iMovie “Corrupted” Frames between two video clips
iMovie is a great free movie editor for Mac. However, an annoying bug is that when two video clips are put together, there can often be “corrupted” frames, glitch, or even choppy video between the two clips.
Simple solution to iMovie Transition Glitch
There are some complicated solutions online, such as re-importing the video clips in certain formats.
However, a simple solution that I tried and tested to work in most cases is to try trimming off a few seconds from the start of the affected video clip. Or even adding a transition effect where the problem arises may work.
To explain more precisely, suppose the problem arises in between A -> B, where A and B are video clips. Then, you can try trimming off a few seconds off the start of video clip B, or adding a transition effect between A and B. (If there is already a transition from A -> B, try removing it!)
Another solution to iMovie Transition Bug
Another solution that I tried and works in some cases, is to export the movie via File -> Share in Best (ProRes) Quality, with Better Quality Compress.

Other solutions to iMovie Bugs between Video transitions
If you have other solutions that you have seen or tried out, please share it in the comments below!