Bring a clip to the timeline. Delete a few seconds or minutes from the start of the clip. Then split the clip and bring the first half (clip A) to the end of the second half (clip B). Grab the start of clip A and drag it to the end of clip B to make a one frame transition. After that, undo it. Instead of undoing the transition clip B will be deleted and the transition clip along with clip A will be pushed back. Demo.
This has happened to me many times for a long time now but it is only recently that I seemingly have found the trigger for it. It’s quite common for me because I will sometimes by accident create a one frame transition when dragging the end of clips and then this very annoying bug happens.