It's usually when I am doing edits. I use S key for split and it doesn't seem to be registering the Split command in the undo list every time (sometimes it works fine). Let's say I select a few events at once with shift+click, then I hit S to split them all, then I move the transport over and do it again. Then I shift+click the middle events and hit Del to delete them. The Edit>Undo list will say:
Undo Delete Event
Now I hit Ctrl+Z
Now the Edit>Undo list will say something else I did before the S commands. The two Undo Split Events are not there (sometimes they are).