Save The Tree / New Teacher - S2-E1
Continuity mistake: New Teacher: Arnold's classroom door vanishes from its door frame in various shots, then the door reappears in its proper place. That door's doorknob also switches between the right side of the door and the left side a few times.
Save The Tree / New Teacher - S2-E1
Continuity mistake: New Teacher: At the start, Principal Wartz writes Mr. Simmons' name on the chalkboard. Then, when Mr. Simmons walks over to the desk, the writing is missing from the chalkboard; it reappears a moment later. (00:12:07)
Save The Tree / New Teacher - S2-E1
Continuity mistake: In the beginning of 'Save The Tree', the tree ladder is a distance above the ground. Later on in the episode, when Arnold and his friends go to the tree house for the last time, the ladder is practically touching the ground.
Save The Tree / New Teacher - S2-E1
Continuity mistake: When Harold eats Mr. Simmons' tomato sandwich, there is a red stain from the tomato on Harold's desk. This stain disappears in later shots.
Save The Tree / New Teacher - S2-E1
Continuity mistake: In New Teacher when the principal is telling the class about their new teacher the students switch seats between shots.
Answer: The show is based in present day times. Plus most cars aren't old fashtioned, I recall seeing a limo and not to mention a cherry picker, etc.