There's most certainly residuals, I've accidentally deleted then installed Windows on top of a bunch of my game saves. I found some random file recovery application and let it run for awhile. Guess what? Nearly everything was readable despite the fact it got wiped and then had a whole windows install.
NAND also experiences minor permanent damage on writes. Actually clearing the NAND involves a write as the charge has to be forced out (a write of 0s)
Nah bro that's just the memory leaks, your supposed to force close and reopen it every so often so the OS cleans up after their shitty application