Yuuri knew better than to enter a room without knocking first.
For starters, it was common etiquette – something that his mother had tried to instill in him. Though, she usually didn't follow the etiquette she talked about, so it isn't surprising that he would forget his manners.
It was just too bad that he would slip and do so while Wolfram and Murata were in the middle of fooling around together.
It was even worse that Murata was dressed like Anissina, and Wolfram like Gisela.
He never did ask them who the lump under-the-covers with the Cheri wig was.