Revealing mistake: When morphing, a "Net" appears over the Rangers, which their Zeo powers then morph them over. Tommy and Tanya's nets don't match their bodies - for example, their shoulders aren't in the net, yet they disappear when their Ranger costume appears.
Monkeyman! / Buses, Bikes and Subways - S5-E1
Revealing mistake: Monkeyman!: When Arnold tells the kids that Monkeyman saved him, as he says, "He went after the two guys who were trying to mug me," he moves his right hand, and it's layered over Helga's hair. (00:01:25)
Helping Hands, Iron Fist - S1-E4
Revealing mistake: When Iron Man appears on the screen in the Hulkbuster armor, if you look at the background behind him, there is a closed door behind him to his right and a control panel directly behind him, which means he appeared from out of nowhere.