The main thing that underflow is getting them is a pointer manipulation which lets them do a stack smash in the ZIP code. Needless to say safe Rust does not allow you to screw up pointer arithmetic and perform a stash smash, even if you can't be bothered to use checked arithmetic and so you underflow.

The equivalent underflow mistake in Rust probably panics. However the RCON trick at the heart of Steam which is powering this lets you run any command as the victim if they accept your "invite" so crashing their game is almost certainly already an option without the underflow.