A single frame from somewhere else in my sequence appears in between two elements, neither of which has that frame.
In one example, it is between two jpegs. In the other it is between a nested sequence and the following MOV.
In both cases, it is a frame for a JPEG that is part of the sequence.
As you right-arrow three times:
1st you see the last frame of the previous element
2nd you see the JPEG
3rd you see the first frame of the next element
(The JPEG is no where near this cut)