When adding tags to a question since enter is the (or atleast an) auto complete key for the inline drop down list if the list isn't visible the enter key will cause the form submit and post the question. This behavior is counterintuitive and more than once has caused me to post my question prior to being finished.
I use this all the time... When I hit "enter" in a form field (with the exception of multi-line input fields), I expect it to submit the form - saves all that effort tabbing to the "submit" button.
The counter-intuitive bit for me is when the drop-down shows up at odd times and enter doesn't submit the form...
Edit: When I originally wrote this, hitting Tab would complete the tag name being typed. You could use enter for this, but tab worked, so why bother...
Tab doesn't work anymore. So enter does two things, and double-tapping might not do what you expect.