Being told to "fix" something, when I'm actually being asked to include a new feature.

This. Also, when 'bugs' are found during testing which are actually new features.

@knuppelsmurf and this

It's always a bug/issue https://bugs.launchpad.net/ubuntu/...

Probably why so many tracking system try to handle it all (bugs, issues, feature requests, tasks, etc.)

Client: "The website doesn't do this (obscure) thing, something must be wrong, you've gotta fix it."

Me: "You never specified that you wanted that feature, I'll have to give you a new quote for the extra work."

Client: "That's absurd! You're supposed to fix bugs like this!"

Me: sigh while thinking about the best way to hide a dead body