“I have too much on my plate right now” - Sometimes, taking on a new project is not feasible. We want our workload to be manageable, but an immediate refusal will not sound good.
"I don't know what you'd do without me" - Saying something like "I don't know what you'd do without me" will make you sound arrogant and like you’re undermining your boss’s capabilities.
“I’m so hungover” - Announcing that you're hungover won’t give your boss a good impression of you. Partying the day before is not an excuse to do a mediocre job.
“Do you mind if I leave earlier to go to an interview?” - While there is nothing wrong with looking for a new job while you’re employed, make sure the process doesn’t impact your current job.
"I could do your job, it isn't even that hard” - Saying you could do your boss' job undermines their position and implies that it's easy.
“You look x today” - Comments on one's appearance are generally inappropriate. Some may be okay, depending on how close you are to your boss, but when in doubt, it's best to refrain.
"Are you pregnant?" - Speaking of inappropriate comments, you should never ask if your boss is pregnant. This can not only cause embarrassment but actually be really offensive.
"No offense, but…" - Saying "no offense, but..." is usually a polite way of saying something offensive and it's likely going to disgruntle your boss.
“Can I borrow some money for x?” - If you need to borrow money for a coffee or something else small, your boss is not the first person you should go to. Instead, ask a coworker.
Slang of any kind - Using slang words when speaking with your colleagues is generally ok, but it's not ok to do with your boss.