When the auto complete dropdown is present you can't use the keyboard to scroll down to the tag you want.
Also if you're typing a tag and it drops down suggestions, none of which are what you want, and continue typing your tag out, the dropdown doesn't ever go away. It just sits there wondering why you're not selecting a tag. This should go away when there are no more suggestions for the user based on what's been typed.
Hopefully some JS ninja's will take this one. I'm the last person you'd want toying with JS stuff.
Peter Sanchez referenced this ticket in commit af4a4e4.