a fly in the ointment
The person who messes everything up when you're trying to have a good time. Like a fly in a jar of ointment, but also a little bit of a jerk.
At the party, my cousin was a fly in the ointment. He spilled punch on the floor and started a fight with my dog.
My friend's ex showed up at our concert, and she was a fly in the ointment. I was having a good time, and now I'm distracted by her drama.
My brother brought a raccoon to my birthday, and it was a fly in the ointment. I was about to eat cake, and now I'm running from a raccoon.