Yes, Virginia, the Mac does have an overwrite key

The general consesus seems to be that there's no equivalent to the windows insert/overwrite on Mac.

Oh, I beg to differ. Strongly. Because there is. I know this because I ocassionally type the overwrite keystroke. I've experienced it in multiple text editors (TextWrangler and Sublime Edit 2) and other places across multiple versions of OS X. Don't believe me? Here's a screenshot of the darn underscore cursor that indicates overwrite:

Yes, Virginia, the Mac does have an overwrite key (or at least shortcut). Will you please tell me what how to undo it?