I'm not sure how, but I managed to get Sublime Text into a 'Replace Mode' which at whatever position the cursor is at, it overwrites the next character with whatever you press instead of inserting it.
How do return back to 'Insert' mode?
I couldn't see any option for it in the menu and searching on Google yielded completely irrelevant results.
For the record, I'm using OS X Mountain Lion, and I have the non-extended keyboard (without the number pad).