Episode Nine - S1-E9
Continuity mistake: When Barnabas enters Josette's room, the door is painted white. When he leaves and Natalie follows, the door has changed to varnished wood. (00:21:15)
Double Date - S4-E18
Continuity mistake: Nick and Carl argue about the incident at the college campus, in May 1970. In the episode "Drivers Education" (season 2), Tony and Joey looked at Nick's high school yearbook from 1970. He couldn't have already been a college student in May of that year.
Continuity mistake: At the start of the baseball game Kevin is holding a baseball bat. The end he is holding the bat changes between shots.
Continuity mistake: When Marshall first finds Trip's letter, he holds it from the left side. In the close-up, he's holding it from the right side. (00:02:10)