I'm working on a custom button where the content is sometimes a textbox. I'm doing this so I have a sort of edit-in-place where you can type text, hit enter, and then the textbox disappears and the button's text is what was typed.
So as a simple case, you could mock this up like so:
<Button>
<TextBox />
</Button>
The problem is if you hit the <space> or <enter> while typing it'll "click" the button instead of input those keystrokes into the textbox.
Now, like I said, this is a custom button so I can do whatever I need to in the codebehind to get this working.