Revealing mistake: In the opening of the movie, a citizen is reading his copy of the New York Post and you can actually read the date; Wednesday September 12, 1990. It is obviously a movie prop though, because the scene was shot in late 1989. How can you tell? Easy; he is reading it next to a newsstand kiosk full of magazines from that period; instantly recognizable are Vanity Fair's November 1989 issue with Baryshnikov on the cover, and GQ from December 1989 with Sting sporting an epic 'stache. (00:00:50)
Continuity mistake: At the beginning of the movie, the stolen wallet ends up in Danny Pennington's hand who is tasked to hold it out for the ninja to grab it. In close-up Danny's hand is positioned below the sleeve of his T-shirt, in the shots before and after his hand is higher, between the sleeve and his body. (00:01:15)
Revealing mistake: In the first, unseen, appearance of the Turtles in the movie, Raphael saves April breaking the streetlight with his sai. The lights go out a good second after the bulb is supposedly shattered, though. (00:03:55)
Continuity mistake: In the beginning, after the Turtles save April, you see her jacket is on correctly as she sits up. A second later, it's off her shoulders. (00:04:25)
Revealing mistake: When the Turtles make their triumphant entrance jumping in frame after the logo, at the bottom of the screen you can tell they are wearing white socks/shin guards, except Raphael who is the one the camera is focused on. (00:05:30)
Revealing mistake: When Don tries to be hip shouting "Bossa Nova!" at the back of his open mouth you can spot the white of the teeth of the actor inside him. (00:05:40)
Other mistake: In the Italian dub, when Michelangelo hangs the telephone reminding the pizza delivery people that "The clock is ticking, dude!" the translation says "Your days are numbered", which is obviously incorrect (the clock is ticking on the pizza delivery time; the guy will be two minutes late). (00:08:05)
Revealing mistake: As Michelangelo is waiting for the pizza guy, Donatelo cames through the sewer on a skateboard. A view of his feet is shown and his hands swipe across the screen. One of the times they come through his hand is not that of a turtle, but a human hand. (00:09:20)
Continuity mistake: As he shouts down to Michelangelo through the sidewalk grate, watch how the pizza man's $10 bill changes from left hand to right in between shots. (00:10:20)
Revealing mistake: When Raph trips the two purse snatchers, the second one doesn't come anywhere near connecting with his leg. (00:11:40)
Continuity mistake: Two shots after introducing himself to Raphael, Casey Jones' hockey mask vanishes from its spot on the grass in the background (full screen version). (00:12:15)
Continuity mistake: After decking Raphael for the Jose Canseco remark, Casey's baseball bat instantly changes hands between shots. (00:12:35)
Revealing mistake: Shredder is enraged because April O'Neil mentioned the Foot clan. He throws a knife at the screen, with an effect that is not only transparent in how fake it is, but shows cracks with secondary circular holes like it was sprayed with bullets. (00:18:05)
Factual error: April interviews (live) the police Chief and then tries to take the train back home. The subway station shows that it's the City Hall stop, and she just finished her interview with the guy, which happened mid-afternoon as you can see by the clock in the corridor. Despite being New York in the middle of the day, there's not a soul in the station, nobody got off the train she just missed, nobody around at all on any of the multiple platforms. It's a trope good for a scene set at 3 AM, not 5 PM. (00:19:25)
Other mistake: The clock behind the couch April rests on (and crawls terrified on when she wakes up) indicates 5:35 for much longer than a minute. (00:21:30 - 00:23:00)
Continuity mistake: When April tries to rationalize what she's seeing, says pointing at the Turtles; "And you guys.. " Mike and Leo are looking at her. Leo's arms are low, but in the reverse shot they are held higher towards her. (00:22:25)
Continuity mistake: When April naively asks "You guys like pizza?" in front of the manhole, her hand drops in the close-up but is still up and pointing in the wider angle. (00:25:40)
Continuity mistake: April is laughing at Mike doing a Rocky impression. Camera behind her shoulder; notice her hand slips off the table. Back to a frontal view, and her hand rests on the table as it was before. (00:26:10)
Revealing mistake: In the scene where the turtles are leaving April's home and stop at the door to say hello before leaving, you can see Leonardo's sword handle bend like rubber, which it in fact is. (00:27:10)
Other mistake: In a close-up of Chief Sterns' desk as he calls the parents of the juvis, you can see (top right) that Danny's arrest date is 9/3/89, which is inconsistent with the date established at the beginning. (00:28:20)
Answer: It's not really petty thievery. Shredder essentially has control over a large crime syndicate which is involved in pickpocketing, burglary, and highjacking. And that's just the gang itself. The Foot Clan itself is probably involved with other activities that we haven't seen.