Continuity mistake: When Barney is knocking on the door at Fred's house after seeing an advert in the newspaper, Fred says "I'll get it, Wilma!" and stops near the door. At this point, Fred is well away from the door, but in the next shot he is now standing near the corner of it.
Continuity mistake: In the scene where Fred and Barney are in the doctor's surgery, the doctor produces a lollipop for Fred. When he produces the lollipop, you can see it's well away from Fred, but as the shot changes the lollipop is much closer to him than it was in the previous shot.