Here was a fun one, several months ago I worked on a track & field pcb that was having some werid issues. The first issue was that the game refused to boot. What this ended up being was a bad interconnect ribbon cable. Easy enough fix.
The second issue was that the game would actually run forever on the 100m. Typically it is supposed to end at some point, however in this case the runners just kept on going, going to a black screen until it timed out. This was a quick fix, the problem was actually a bad ROM at 5A (I think that was what it was, I don't quite remember because it has been a really long time since I did this fix).
Glad it was a pretty quick fix, ROMs are one of the easier things to diagnose if you have a programmer. A quick reprogramming of the ROM completely solved the issue and the game worked as it was supposed to.