2. Overdoing it
My husband does this a lot. He has a way of getting in a bad mood when the kids are fresh (imagine that!), and he metes out punishments that are either more reflective of his mood than the crime or thoroughly unworkable, like saying "You have to stay in your room this afternoon" when he has errands to run and needs to bring the kids with him. The punishment should fit the crime, not your frustration level. And it needs to be something feasible, that doesn't overly affect siblings who've done nothing wrong.
A friend taught me a great trick. If one of the kids is doing something he shouldn't -- being mean to a sibling, for example -- I say, "There will be consequences." (It's particularly good to use in public, because while it may strike fear into your kids, it sounds pretty benign.) Over the years, it's been shortened to "Consequences!" with the appropriate firm-but-not-yelling voice, furrowed brow, and I'm-totally-serious gaze.
If the misbehaving child doesn't stop, there are consequences, but I have a moment to think about them. Sometimes I'll ask, "What do you think your consequences should be?" It's interesting how often kids come up with a fair punishment (e.g., apologizing and letting the wronged sibling play with his favorite toy for the rest of the day).