Revealing mistake: In every episode, the windows in both the kitchen and living room areas never reveal any scenery outside them during the day or any reflections from the inside at night.
Continuity mistake: When the band is sound-checking just before the party, the clock behind Trent has no hands on it. Then when the party starts, the clock has hands once again.
Continuity mistake: Daria arrives at school without her glasses (her choice). When the scene changes to Mr. O'Neil's class, first shown from the back facing O'Neil and the backside of students, Daria (sitting on the left of the screen) is shown from behind and her glasses are showing on her face (as usual). When Daria is next shown talking close up in the remainder of the scene, she is without her glasses again.
Continuity mistake: In the Season 1 episode "Too Cute," Quinn mentions to Sandy that Sandy picked out Quinn's outfit that she's wearing. This could not be possible as Quinn had that outfit (the same one she always wears in every episode) since BEFORE they moved to their current town and before the girls met anyone in their school they first went to in the first episode.
Continuity mistake: In many episodes throughout the series, whatever is written on the blackboard behind a teacher in a classroom always changes many times in different shots. Some words or pictures appear or disappear or change shape, features, etc.
Esteemsters - S1-E1
Continuity mistake: When Mr. O'Neill is talking to Daria and Jane, the board behind him has a bunch of arrows in a circle with a different word written next to each arrow. When the camera changes to a further angle with all three characters in the frame, the arrows remain on the board but all the words are completely gone - then appear again once the camera is close up to the board again.
Continuity mistake: When Jane (who has blue eyes) first meets Tom and talks to him, her eyes become green at one point, then become blue again.
Continuity mistake: When Daria is on the couch with her dad she's holding a mug but the next shot it's a glass. (00:15:39)
Continuity mistake: Throughout the episode, the girl in Jane's painting rests her head on her hands. In her reflection, her hands are turned outward on her chin even though she is sitting there with her hands turned inward.
Continuity mistake: Inside the pizza restaurant, the front door opens on the opposite side compared to how it's designed from the outside.
Other mistake: In one scene in this episode, the clock on the kitchen wall (behind Jake) does not have any hands.
The New Kid - S2-E7
Plot hole: In the season two episode 'The New Kid,' Daria and Jane are discussing The New Kid in gym class when a girl takes a nasty spill from a trampoline. Later, Daria and The New Kid are heard remarking about photos taken of that particular fall despite there having been no photographers present.