If you find yourself using a keyboard without an Insert key (eg recent Mac keyboards) but Overtype mode has enabled itself somehow, then it's useful to remember that the 0 key on the Numberpad is a surrogate Insert key, but only when Num Lock is turned off. If you have NumLock set On and the NumLock key disabled, though, this is a small problem: KeyMapper lets you toggle the value of Num Lock from the Toggle Lock Key menu, so you can set it off temporarily, switch back to Insert mode using Numberpad/0, then toggle Num Lock on again.