When Jack is negotiating with David, Dutchy and Swifty are standing in the background of the group behind David. In the next shot, they are coming down the stairs to join the group.
When Medda is sing "My Lovey-Dovey Baby" it show Les sitting by a man bouncing a puppet on his Leg. In the next shot, the man with the puppet sits down by Les.
When Joesph Pulitzer was reading Trolley Strike Drags on for third week, he drags his magnifying glass and reads the top line but says the word 'on' (which is on the second line) while he's still reading the top line.
When David goes to rescue Jack from the refuge, he removes a pin to uncouple the horse from the carriage. He's leaning on his right elbow, and the pin is above his head, yet he uses his right hand to reach up and push the pin up, and then he grabs it from above with his left hand. There is no way he would be able to do this from that position, as he could not prop himself up and push the pin out with his right and at the same time.
When Jack, David, and Les go to their house, Sarah picks up a fork before gives Jack the plate with the cake. In the next shot she has to pick up the fork off the table and give it to Jack.
The real 1899 New York newsboys' strike was a partial failure. Prices were not lowered, although before the strike, unsold papers were not refunded, whereas afterwards they were.
By 1899 Joseph Pulitzer was blind and ran his newspapers by telegram from a yacht.
While Joseph Pulitzer did have a daughter named Katherine, she died in 1884, 15 years before the setting of this story.
A hand-operated platen press can produce 250 sheets an hour. In eight hours (a generous estimate of the time they had) that would be 2000 sheets, yet the newsies distributed far more than that.
When the Newsies line up at the gate during "Carrying the Banner", most are singing the first verse; it's not obvious, but Kid Blink is singing the one that is sung in counterpoint.
At the end, when David puts on his hat, someone bumps the camera causing it to shake.
During her fight with the Delancey brothers, when Sarah pulls back to punch one of them, her fist hits the "brick" wall and gives way.
When Boots and Les sing their solos in "King of New York", you can see the hands on the backs of their chairs that push them up.
During the toast to Denton, in the background, you can see Mush and Kid Blink (Aaron Lohr and Trey Parker) standing together. When the drinks are being handed out, Racetrack (Max Casella) takes two, and hands one back to Trey and keeps the other. Trey hands the drink to Aaron and then taps Max on the shoulder for another drink, but receives none. He then holds the same glass as Aaron, and the two laugh about their toasting with the same glass.
In the director's commentary they comment on the real outcome of the strike. Indicating that they were aware of the reality, but probably wanted to simplify the ending.
In the rally scene, there is a brightly lit EXIT sign in the background.
After talking to Jack-as-scabber, Davey doesn't move his lips when he says, "I'm not like you!"
When Jack, Davey, and Les are being chased by Snyder, Les's lips don't move when he says, "All this for one sip of beer?"
During "Seize the Day" the kid with the tin can drums is off beat compared to the soundtrack.
In the beginning song "Carrying the Banner" when Crutchy sings there are unsychronized voice/mouth movements.
When the Newsies are fighting the men sent by Pulitzer, Racetrack throws a punch that is not accompanied by a sound.
When Jack, Davey and little brother are running into the theater. As they round the corner there is a palm tree. Probably not something seen in NY in the 1890s