Shit in the mikveh

[Tina]

To spoil an otherwise successful endeavor or to ruin an otherwise positive moment. A sort of [haimish] or [heimish] (i.e., Jewish-inflected) version of turd in a punch bowl or turd in ones drink. A [mikveh] is a ritual bath used for purification and conversion in the Jewish tradition and, thus, shitting in one is strictly forbidden.