When Oscar & Felix are arguing in the bathroom, Oscar still has residual shaving cream on the left side of his face. When the camera cuts to a different angle, the shaving cream is gone.
Myrna comments on the large "Thanks" poster Felix gave to Oscar. Oscar says he gave it to him after he let him move in. Actually, Felix gave it to him for saving his life in "You Saved My Life" (S2 E17).