With extremely rotted (but still technically playable) disks, in many cases players will lose TBC lock, leading to entire lines losing correct colo(u)r and horizontal jumps in the picture. This seems to me like something that should be added to level 4 at least as "picture distortion and/or horizontal color lines"
https://archive.org/details/ASPEN4 is a good downloadable example. Unfortunately it's currently the only known copy of the disk, but a silver lining is that there's an RF capture, and ld-decode can keep sync most of the time on it. (an ld-decode copy is on youtube but heavily compressed there)