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 clincher.Getting caught in a downpour was the clincher on a terrible day.
clincher
(ˈklɪntʃɚ)
n. the final element; the straw that broke the camel’s back. (see also capper.) The clincher was when the clerk turned up the volume.