spit or piss into the wedding cake  {v. phr.},  {vulgar},  {avoidable}
To spoil someone's pleasure or celebration by doing or saying something harsh or unseemly in an otherwise happy gathering; bring up depressing or unhappy subjects at a supposedly happy time.
Stuart really spit into the wedding cake when he told Burt in a bragging fashion that Lucy, Burl's bride, used to be his girlfriend.
Categories: pleasure time {avoidable} {v. phr.} {vulgar}