There is a #xmpp client named #gajim. They have keyboard shortcuts. One of them is CTRL U to delete the currently typed message.
First: Why the fuck would anyone want a keyboard shortcut for that?
Second: I often use CTRL SHIFT U to enter Unicode characters. You'll never guess what happens every now and then…
Oh, and CTRL ↑ edits the previous message, and in the process, also deletes the message I'm typing. I use CTRL ←/→ to jump between words, and … yup.
What is your favorite XMPP client?