Lisa looks extremely odd in this scene from Season 12's "Lisa the Tree Hugger".
These are the courtroom scenes from 'Homer to the Max'. In this first one, you'll notice the faulty crowd animation.
Agnes in particular is horrendously off-model in this scene - here are two frames of her:
In this next one, Agnes' eyes are randomly enlarged for a few frames, then return to their original size. You have to save these pictures to your computer and flip past them both to see any noticable change.
The "mistakes" in Rome-old and Juli-eh aren't just exclusive to that episode, animation like that can be seen throughout the entire digital era (another episode where it's hugely noticeable is Kiss Kiss, Bang Bangalore). It's caused by the camera being zoomed in too far over the cell while they are filming (before the colouring process is done), allowing the uneven-ness of the inked line to become apparent. This usually occurs in scenes where the camera zooms out during the shot, which results in the lines finally smoothing out.
Two huge ones from Season 10; - During a court scene in When You Dish Upon A Star, Bart's head is briefly missing from his body. It suddenly reappears in the middle of a scene. - When Marge is watching Bart going through her purse in Screaming Yellow Honkers, most of Bart's body disappears for a few seconds, leaving his head and arms floating above his legs.