[7kay]

One other method to quickly exit insert mode if you can't remap keys is ^C. I did not know this even though I used gvim on Windows computers not under my control for years.

[alexjm]

True and handy. However, Esc and ^C don't have exactly the same behavior -- from the documentation:

<Esc> or CTRL-[ End insert or Replace mode, go back to Normal mode. Finish abbreviation.

CTRL-C Quit insert mode, go back to Normal mode. Do not check for abbreviations. Does not trigger the InsertLeave autocommand event.