As far as i can see all the options that could cause this are off, so i have no idea where else to look - theres nothing in the autocomplete options, or the language settings etc, so.......
edit: sorry - after looking for an hour, then posting this, i realised what it was about 10 seconds later - i had the same data higher up the page, so when i was clicking enter it was thinking i was clicking to accept the autocomplete