Graduation - S5-E24
Factual error: In the episode Class Pre-Union, they were the class of 2000. In the episode Graduation, they were the class of 1998.
Return of the Green Ranger (2) - S2-E45
Factual error: Billy, Kim, Rocky, Aisha, and Adam return to Angel Grove in the 1700's...and there are British Redcoats there. On the West coast, in California. England never deployed troops to California, especially considering it was part of the empire of Spain at that time.
Factual error: None of the Swedish wedding traditions Ingrid mention actually existed in the 19th century and the bread she's baked is not traditionally Swedish. When she later speaks in her mother tongue it's definitely not Swedish.
Factual error: Ben Franklin's "electricity kite" experiment and the Declaration of Independence happened 24 years apart, not at the same time.