Factual error: When the police chief is looking at Danny's police file just before calling his father, you can see a few different phone numbers listed. These numbers are all using 213 as the area code which is for Los Angeles, CA. Given that the film takes place in New York City and that the addresses listed on the same police file are NYC addresses, the phone numbers should be using 212 area codes. (00:28:22)
Audio problem: In the Italian dub, Chief Sterns reads Mr. Pennington's number as 555-5219 instead of 555-5214. (00:28:30)
Visible crew/equipment: As April walks from the bathroom trying to get Charles off her case, the top of a camera operator's head is briefly reflected in a mirror above the aquarium. (00:29:50)
Continuity mistake: While driving Danny to school, Charles' left hand grips the steering wheel. But in the following shot where Danny opens the door to run away, his left hand is resting near the window sill instead. (00:30:25)
Continuity mistake: After Master Tatsu kicks his opponent in the face, a couple of bystanders in the background instantly move further apart from each other between shots. In addition, one of them is a Foot member who loses his nunchaku as the shot changes. (00:32:30)
Visible crew/equipment: In the shot in April's apartment where Leonardo and Raphael are arguing about how they should look for Splinter and the Foot, a crew member with an orange hat can be seen ducking behind the table on the left. (00:38:00)
Continuity mistake: After Raphael has a fit and walks out of April's apartment, he goes up on the roof to vent his anger for a while. Pretty soon, Casey spots him from another rooftop several buildings away while tuning a radio. As Casey looks out at Raphael's faint image in the distance, pay close attention to the background as well as the white building Raphael is standing on (you can notice the Empire State Building among other landmarks). When Casey grabs some binoculars to take a better look, Raphael can be seen resting on a brick ledge. In the following shot Raphael looks out at the city towards Casey's vantage point, but from Raphael's position, the Empire State Building - as well as the white building where he was venting just moments before - are visible on the skyline. Somehow Raphael has managed to relocate to another building's rooftop between shots. (00:38:15)
Revealing mistake: When Raphael is fighting the Foot Clan on the roof of April's building, after he tells them that they must be studying the abridged book of ninja fighting, he kicks one of them in the stomach and then cartwheels over his back to kick another one in the face. You can tell he misses the second one by a mile, but the man still groans and collapses unconscious into the chair behind him. (00:40:45)
Continuity mistake: In the rooftop fight, as Raphael quips about the Foot guys studying the abridged version of the Ninja book of fighting, you can see his belt is hanging kinda low, at the hips almost. In the next shot, Raph is fighting with his belt in senior citizen mode, pulled all the way up to 2/3 of the torso. You can tell quite easily because being a turtle his belly is divided in sections. (00:40:45)
Revealing mistake: During the fight sequence in April's apartment, two foot soldiers dunk Donatello into a fish aquarium. Instead, you can see how it's actually a steel tub full of water into which he's dunked (fullscreen). (00:43:10)
Revealing mistake: In the middle of the fight inside April's apartment, there's a comedy/action bit with Leonardo repeatedly trying to slash at a Foot Clan soldier. Ultimately, he fools him and kicks him. Behind him, another two bad guys arrive, but there's a very awkward bit in the choreography where one of the guys quite literally does not know where to go and puts himself in a corner with nobody there, stumbling into the other. (00:43:30)
Continuity mistake: Mike thanks to Don's help does the "Wheel of Fortune, dude!" move; the floor next to him is relatively clear from debris and he sweeps it, but when the bad guy throws a single axe strike next to his head, the floor is all messed up and cut. (00:44:00)
Continuity mistake: During a fight sequence between the Foot Clan and three of the turtles in April's apartment, keep in mind that April is still tending to an injured Raphael in the living room. But as the floor collapses into an antiques store below, you'll be able to spot her on the lower left side of the screen shielding her eyes from debris - instantly in the store before anyone else (except for Master Tatsu and several other Foot members, who were waiting there to begin with). (00:44:15)
Continuity mistake: In the rumble inside the antique store, Mike at one point squeals that they could use Raph, "right about now." Cut to Donatello fighting some bad guys to the left of an unimpressed and passive Master Tatsu. The following shot has Don fighting Foot clan members to Tatsu's right, and it's Leonardo to be battling to Tatsu's left (right side of the screen). (00:45:30)
Continuity mistake: Casey Jones arrives to the save the day. Michelangelo does a Wayne Gretzky reference. Notice in the background Master Tatsu, solitary and surrounded just by old abat-jours and other dusty curio. When he orders in his spartan English to "attack", he suddenly has a bunch of goons all over him. (00:46:05)
Audio problem: While Casey fends off the Foot Clan in the antiques shop, an answering machine falls from April's apartment above, hanging by its power cord. Charles' voice can be heard on the speaker, but after the cord snaps his voice can still be heard finishing a sentence. (00:46:50)
Continuity mistake: Master Tatsu is raging against the students, and slams one against the lockers. Another Foot soldier tries to restrain him. The unmasked victim of the assault is clutching the shoulder bit of Tatsu's hood, but his hand is further down his arm in the reverse shot. (00:49:00)
Other mistake: April mentions that she has not been to the old farmhouse in years, and that the nearest neighbour is four miles away, but the morning of their departure is signaled by a rooster crow. (00:51:00 - 01:01:50)
Other mistake: After Michaelangelo hums to agree with Donatello's comment moments after Casey & April have finished arguing and slamming doors, you can hear Michaelangelo humming "Old MacDonald," but you can see a shoulder strap and a purple headband indicating this is Donatello. (00:52:15)
Continuity mistake: In the alphabet game of insults, Donatello calls Casey Jones "elf lips." Casey puts the monkey wrench on the radiator next to the dirty rag, but in the next shot the tool is gone from there and is in his left hand instead, so he retrieves only the rag from under the hood. (00:53:10)
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.