mikveh

a bath in which certain Jewish ritual purifications are performed.

The jews decided to take a mikveh

Shit in the mikveh

"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 one's drink." A mikveh is a ritual bath used for purification and conversion in the Jewish tradition and, thus, shitting in one is strictly forbidden.

Me and my buddy were having a great morning shooting out on Forest Service land until a couple of noobs with zero target acquisition skills and good sense showed up and took a massive shit in the mikveh by sending a bunch of 5.56 across our baffles.