I tried twice this week to create a mosaic, and both times the exact same error happened.
All the subs lined up well, except one.  Is this a common issue?  What can I do to prevent this?

(These were just single shots, no pre processing, flats darks etc.  I wanted to make sure I was able to do this before I committed a lot of time.)

(C11 EdgeHD, f/7, ASI1600MP, CEM120 EC2)

 
 

Seems you have a bit too large tolerance for alignment error compared to overlap in mosaic planner. I would recommend at least 20% overlap (also helps blending to hide seams between the panels if necessary) and possibly a bit less tolerance for alignment so that the panels are more in line (though dithering will make them jump around a bit anyway, but less so)