For instance, you can only use "the eraser trick" on pixels placed by floodfill. It won't work with pixels placed by anything else (even the damned PENCIL), which is a major inconvenience I had to endure.
OH MY GOD. IS THAT WHAT IT IS?! I haven't been able to work that out at all! I couldn't figure out why it would work sometimes and not others! WTH Paint!