[im3w1l]

Undefined behavior has a very specific meaning for C. It doesn't mean "not 100% precise". It means 100% imprecise. The C standard only gives any guarantees on what your program will do if you never invoke UB. If you do, well then it can do literally whatever it wants including deleting all your files.

[MaulingMonkey]

Literally deleting all your files at that. It's the part of the language that says your compiler is completely justified in allowing you to write that buffer overflow vulnerability that can trample executable memory, which is then used by a malicious attacker to do literally whatever they want, including deleting all your files. Or worse.