Continuity mistake: The age gap between Eric and Cory decreases as the series progresses. At the start of the show, Cory is in 6th grade and Eric is a high school sophomore, meaning they are four years apart, give or take a few months. By season 3, Cory is in 9th grade (confirmed by dialogue mentioning that Shawn, who is in the same grade as Cory, is 15 years old), and Eric is a senior, leading to an age gap of three years. The first episode of season 4 takes place a couple months after the end of season 3, with Eric and Cory returning from a summer Road Trip, but just two episodes later, Cory mentions that he is in 11th grade, while Eric is still barely out of high school, leading to an age gap of two years.
Green With Evil (1): Out of Control - S1-E17
Continuity mistake: Up until Green No More in Series 2 (where the Green Ranger powers are destroyed), the Green Ranger's overshield constantly changes between solid-looking metal to flimsy-looking cotton. This is the most obvious difference between the Japanese and US footage.
Best Friends - S2-E9
Continuity mistake: Near the beginning when Colleen and Becky are talking to the boy, there is a boy walking in the background. When he loses his hat, he is behind the boy talking to the girls. In the next shot he is already behind the two girls, which is quite a great distance to cover in a split-second. (00:06:30)
Continuity mistake: When the Warners emerge from a sealed crate on Miss Flamiel's desk and start singing, Yakko instantly appears behind Wakko and Dot as soon as the shot changes.