Last night my mount (A Losmandy Titan 50 with Gemini I L4 controller) hit the safety limit and stopped imaging, see log below.

I was taking an 1800 second exposure of IC405 which crossed the meridian about 10:20 UTC according to TheSky.

I thought I had this figured out. The safety limit is 5 degree West of meridian, which I calculated to be +20 minutes of time (HA). It flips on a goto at +10 minutes past meridian. I verified these from the manual (see also below) and by experimentation. Hence in ACP preferences (GEM flip settings), I set "When slewing East to West, mount flips 10 minutes after meridian,"Mount can track 10 minutes past flip point," and "Mount needs 4 minutes to assure flip"

ACP log follows.

ACP console log opened 01-Nov-2018 02:12:48 UTC
This is ACP version 8.1 (build 2, Product Version 8.1.1)
Licensed to Michael Vander Vorst
02:12:48 Logging to C:\Users\Public\Documents\ACP Web Data\Doc Root\logs\mvandervorst\20181031\20181101@021248.lo g
02:12:49 Initializing AcquireSupport V8.1.1
02:12:49 Telescope is ACP->Gemini Telescope .NET, driver V2
02:12:49 Dome is Seletek Dragonfly
02:12:49 and must not close unless scope is parked
02:12:49 MaxIm DL is version 6.18
02:12:49 Imager is FLI
02:12:49 Using focus offsets and star mags from FilterInfo.txt
02:12:49 Selecting filter Luminance from previous ACP run at 20181028@021243 UTC
02:12:49 Imager readout modes:
02:12:49 0 is 8 MPPS High Speed
02:12:49 1 is 8 MPPS High Speed (RBI Flood)
02:12:49 2 is 1 MPPS Low Noise
02:12:49 3 is 1 MPPS Low Noise (RBI Flood)
02:12:49 Using Normal and Fast readout modes from RoModeInfo.txt
02:12:49 Normal mode is 1 MPPS Low Noise (RBI Flood)
02:12:49 Fast mode is 8 MPPS High Speed
02:12:49 Autoflat mode is 1 MPPS Low Noise
02:12:49 Guider is QSI Universal (no rotator)
02:12:49 Guider plate scale is 1.7 arcsec/pix. Guided dither with main imager pixels
02:12:49 Max unguided exposure 30 sec.
02:12:49 (assuring that FocusMax is running now...)
02:12:55 Calculated unbinned plate scales (arcsec/pix): H = 1.71 V = 1.71
02:12:55 Calculated field of view (arcmin): H = 117.0 V = 117.0
02:12:55 Plate solving With GSC 1.1 (ok for wider fields)
02:12:55 Mount supports tracking rate offsets
02:12:55 All-sky plate solving will be attempted when needed
02:12:55 All-sky service at nova.astrometry.net will be used
02:12:55 This is AcquireImages V8.1.0
02:12:55 Hardware pier side reporting is available.
02:12:55 This mount can be flipped on command.
02:12:55 Compiling plan...
02:12:55 (using ACP Plan Compiler V8.1.0)
02:12:55 ...plan OK!
02:12:55 == Resuming interrupted plan ==
02:12:55 This plan has 7 live target(s), 122 images
02:12:55 There are 1347.5 min. of actual imaging time
02:12:55 Image file set-numbers start with 1
02:12:55 Periodic auto focus every 135 min.
02:12:55 (includes a plan-start auto focus)
02:12:55 Starting run for plan IC1396NB+IC405.txt...
02:12:55 Autofocus will be done every 135 min.
10:19:47 [flip check: Tn=1200s HAc=-43s GW=F HAz=1161s DWz=F WF=no]
10:19:48 (long exp(s) or requested, no orbital tracking, trying to autoguide)
10:19:48 Switching from Luminance to Si filter for imaging
10:19:48 Focus change of -46 steps required
10:19:56 (take guider image at initial interval 5 sec)
10:20:43 (guide star SNR=very high X=1739 Y=1598)
10:20:43 (first exposure estimate 4 sec)
10:21:30 (guide star SNR=very high X=1741 Y=1599)
10:21:30 (final exposure for guiding 4 sec)
10:22:17 (guide star SNR=very high X=1742 Y=1599)
10:22:28 (autoguiding at 4.00 sec.)
10:22:28 === Place in plan ===
10:22:28 Set 1 of 1
10:22:28 Target is "IC 405" (5 of 12)
10:22:28 Repeat 1 of 2 for this target
10:22:28 Filter Si (1 of 1)
10:22:28 Image 5 of 9 for this filter
10:22:28 Imaging to IC 405-S001-R001-C005-Si
10:22:28 (taking 1800 sec. exposure, Si filter, binning = 1)
10:22:28 (using 1 MPPS Low Noise (RBI Flood) readout mode)
10:22:35 (ex=0.17 ey=0.28)
10:22:35 (guider check OK)
10:24:32 (starting exposure)
10:54:32 (exposure complete)
10:54:57 (exposure complete and image downloaded)
10:55:01 Image finished
10:55:24 Plate-solve final image
10:55:34 28988 image stars found
10:55:35 4488 catalog stars found
10:55:38 Solved! 395 stars matched.
10:55:38 Average residual is 0.39 arcsec.
10:55:38 Pointing error is 0.412 arcmin @ angle 294.08
10:55:39 True focal length is 1081.0 mm.
10:55:39 True binned plate scales (arcsec/pix): H = 1.72 V = 1.72
10:55:39 True image center (J2000): 05h 16m 28.2s 34° 21' 10.09"
10:55:39 Imager sky position angle is 353.4 deg.
10:55:39 Image FWHM is 5.1 arcsec (2.95 pixels)
10:55:39 Within max error tolerance, no recenter needed
10:55:40 [flip check: Tn=1200s HAc=1506s GW=F HAz=3319s DWz=T WF=YES]
10:55:40 (stopping the autoguider)
10:55:40 (GEM must flip before next operation)
10:55:40 [FPw=600s FPe=-1200s HAt=2119s PFM=240s
10:55:40 (no wait needed, already past flip limit)
10:55:40 (flipping mount...)
10:55:40 Start slew to IC 405...
ACP console log closed 01-Nov-2018 10:55:53 UTC

From Gemini manuals

Gemini manual

3.3.4 GoTo Commands and Meridian Flips
By default, if you attempt to GoTo an object (using either the menus on the hand
controller or a command transmitted via the serial port), Gemini will attempt to point to
the object without doing a meridian flip if the safety limits allow it. If the telescope is on
the east side of the mount and the object can be reached without moving past the east
safety limit, Gemini will point without a meridian flip. Similarly, if the telescope is on
the west side of the mount and the object can be reached without getting closer than 2.5
degrees from the western safety limit, it will point without a meridian flip. In all other
cases, the mount will do a meridian flip before pointing to the object.
For example, if the western safety limit is set to the default value of 122 degrees from the
CWD position, a GoTo command to any object from the eastern horizon to 29.5 (122 –
90 – 2.5 = 29.5) degrees west of the meridian will not cause a meridian flip. This could
be a problem if you want to start imaging an object that is, say, 25 degrees west of the
meridian. If the telescope is on the west side of the mount, a GoTo command will not
cause a meridian flip since the object can be safely reached without it. However, you
probably want to perform the meridian flip before you GoTo this object to give you more
time to image before reaching the western safety limit. If you are controlling the
telescope manually, you can use the QuickMenu “Meridian Flip” command to force a
flip. Alternatively, if you are using telescope control software that understands Gemini’s
serial commands, it could check which side of the meridian the mount is on, and issue a
meridian flip command. However, this does not work with most control software that
uses only the Meade command set without Gemini’s extensions. For these programs, the
only solution is to GoTo an object that is past the western safety limit to force the
meridian flip, and then go back to the object of interest.
To solve this problem, Gemini now allows you to set a “GoTo Limit” to define a point
past which Gemini will always GoTo the object so that the telescope tube is on the east
side of the mount. Gemini will allow the mount to track past this point, but will not
GoTo any object west of this point without a meridian flip, if needed, to put the telescope
tube on the east side of the mount. Refer to section for a description of the
“Set GoTo” limit command. Set Safety Limit
Set GoTo:
The “Set GoTo” command allows you to specify an angle relative to the CWD position
which will affect meridian flips during a GoTo operation. A GoTo command to any
object that is west of this angle will cause Gemini to do a meridian flip, if needed, to
ensure that the telescope tube is on the east side of the mount when pointing to the object.
For example, setting the GoTo limit to 95 degrees means that a GoTo command to any
object 5 degrees or further west of the meridian will cause Gemini to point to this object
with the telescope tube on the east side of the mount (doing a meridian flip if needed).
The telescope will still track an object past this point if the tube is on the west side of the
mount, but once an object has passed the GoTo limit, a subsequent GoTo command to
this object will force a meridian flip.
A value of 90 sets the GoTo limit to the meridian itself; meaning that a GoTo command
to any object west of the meridian will force the telescope to the east side of the mount.
The default value of zero means that the GoTo limit is set 2.5 degrees east of the western
safety limit.
Note: Use caution when setting the GoTo limit to values between 1 and 89 degrees.
Such values would indicate points east of the meridian, and would mean that Gemini
would try to point to objects between the GoTo limit (now in the eastern hemisphere) and
the meridian with the telescope tube on the east side of the mount. It is possible that such
objects would be unreachable without violating the eastern safety limit. Attempts to
GoTo such objects in this situation would result in the telescope not moving, and
“*Interrupted*” being displayed on the hand controller.

Gemini.net manual
Goto Limit – The Goto Limit is used by Gemini to determine if a meridian flip is required for a Goto
command. The default value of zero means that Gemini will set the Goto limit 2.5 degrees before the
relevant safety limit.
Automation Settings – Lastly the Automation Settings is a brand new feature of this driver. Normally if
Gemini tracks into a safety limit the mount stops and has to be manually backed away from the limit
before you can Goto your next target. Whilst this is a minor inconvenience for someone local to the
mount, it is a major problem for automated operations.
If this parameter set it invokes a one second slew, at the currently configured slew rate, away from the
safety limit. This feature is used by all ASCOM slew commands, including Alt/Az slews and equatorial
slews. It is also used when a park command is issued that requires the mount to move from the current
position. The driver waits for the one second slew to complete, and then issues the desired slew/park
command, as usual.