A shit in the punchbowl
Everyone’s having a good time, but that one person acts like they own the punchbowl and decides to poop in it.
The birthday was going great until Tim showed up with a bag of chips and started eating them like they were the last meal on Earth.
We were all singing and having fun until Lisa started arguing with the DJ over the music choice.
The game was going well until James spilled his drink all over the floor and nobody could move.