When the character of Norm walks into the bar, he pats another bar patron - who is sitting at a table near the bar entrance - on the shoulder while on his way to his regular seat at the end of the bar. When Norm arrives at his seat, that same bar patron is now seated at the end of the bar.
Just before Norm calls Sam a lush face, his mug is completely empty, the picture cuts to Sam then back to Norm who's mug is now half full without being given a new beer.
The number "3" on Diane's apartment door is either upside down or mounted backwards.