If you have an internet style signature, and you compose a new email, then tab key from subject to the body of the email, the cursor is placed *below* the signature rather than at the top of the message (where you will start typing), so you always need to arrow-up to get the cursor positioned properly.

This is an old bug that never got any love

How about some votes: http://bugzilla.zimbra.com/show_bug.cgi?id=1457