Rebasing a branch which has already been published or merged from (and then re-publishing that branch). Doing so will make everyone hate you as it will result in awful problems since you've just rewritten history and require those people who have merged from your pre-rebase branch to fix the problems introduced by your rebase manually.
Also see http://hades.name/blog/2010/03/03/git-your-friend-not-foe-vol-4-rebasing/ for details.
