Joe Choynski is shown as a big brute. The real Choynski was a middleweight who never weighed over 155 lb., and was much smaller than Corbett.
There was no "Victoria Ware" in real life. From the start of his boxing career in 1886 until 3 years after his fight w/ John L Sullivan, Jim Corbett was married to Mary Higgins. After their divorce, he married Vera Taylor.
In the fight scene at the beginning of the movie, when the police swarm in and begin beating the two fighters with their night sticks, you can clearly see several of them flexing as they are being swung. This is particularly evident with the officer on the left as he repeatedly hits "The Mauler". It's obvious they are made of rubber.
In the bank the day after the first fight (and their arrest with the bank director for attending it), Corbett (Flynn) and Lowrie (Carson) decide they should resign preemptively, rather than wait to be fired. However, Corbett gets a raise instead and returns from his boss's office to tell his pal Lowrie about it. As he talks, he's counting a stack of "money" of which only the top and bottom sheets appear genuine. The other "notes" are all blank sheets of paper.
In the second round of the championship fight, when Flynn (Corbett) is against the ropes his timing is off and he begins to duck before Bond (Sullivan) starts his punch. Bond continues his punch anyway, although a real boxer would have stopped and punched down where Flynn was.
As the referee counts one of Sullivan's knockdowns, the soundtracks says, "One, two, three, four..." while his lips obviously count, "...four, five, six, seven."
The night the Corbetts are moving out of their old home to go to the one that Jim had bought for them, the shadow of the boom mic is clearly seen moving along the left wall as they come down the outside steps.