Highest z-index was an astonishing 999999999999999999999999999 or 9.99e26. With a default key repeat on OS X, this would take 3 seconds of holding down 9 to type. If you took that many pieces of paper at .05mm thick, the stack would reach from Earth to the Sun 10 trillion times. The worst part about this large of a z-index is that a number that high will overflow and not work in the expected manner anyhow. The lowest was a more reasonable -999999.
FWIW, max z-index that is cross-browser safe is: 16777271
I think we've all been there and started throwing out crazy z-index values at one point or another, usually out of frustration. The only real solution is to keep this stuff under control from the start and always use exactly the z-index you need.
Ahaha totally. I can just imagine the hilarious scene where a poor dev just breaks after a solid 45 minutues of trying to get that god-forsaken div to overlap another and gets to his feet, punchs down the 9 key for a whole 3 seconds screaming a 3-syllable combination of rage-soaked curses ("cock-sucking cunt" comes to mind), finally collapsing back into his seat on the last; too pissed to even bother hitting Ctrl+R to see if it had worked, typing 'git add . && git commit -m "fuck everything." ' before going home.
FWIW, max z-index that is cross-browser safe is: 16777271