I usually fix any problem right away. However, when I'm planning to rotate out a game and it developes a problem, I write the problem on the playfield glass with a felt pen. That way, when I stand up the game again later to play or sell it, I know what I need to do. Writing stuff down on paper is a waste of time for me, I'll lose it before I need it. I might add that using a felt pen on PDI glass is probably a bad idea.
Post edited by blownfuse : adding info