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.
Revealing mistake: It is very easy to tell when Salem is a real cat and when Salem is a robotic cat.
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.
Episode #1.2 - S1-E2
Revealing mistake: Towards the end of the episode when Dinah, Lloyd and Harvey are walking to school you can see a man and a woman stood at the end of their driveway watching filming. (00:19:28)
Take Me out to the Ballgame - S4-E3
Revealing mistake: When the fat guys are running down to stop the bad guys, you can see the blue safety mat that they land on just after jumping. (00:37:00)