The final thing that completes something, possibly in a negative way. I'd been having a wonderful birthday already, and being surprised by my whole family at dinner was the capperGetting caught in a downpour was the capper on a terrible day.
capper
(ˈkæpɚ)
n. the climax or clincher of something. The capper of the evening was when the hostess got lathered before midnight and couldn’t celebrate the New Year.