Other mistake: In the scenes after Giovanni Ribisi is arrested, his looks are constantly changing. In scenes where he is talking to Vin Disel his face is very fat. When he walks by Abby's desk he is thin again. If you get Boiler Room on DVD you'll see an original ending where Ribisi is thin all the way through the end of the movie; Because Giovanni was preparing for a film that required him to gain 30 something pounds, the scenes that needed to be reshot for a different ending are very defined.
Other mistake: In the scene where all the guys are in a bar and Greg is about to leave with Seth, a big guy bumps into Greg. When Richie is beating this big guy up outside the bar in the alleyway, you can see that the guy is fully conscious but is letting his arms just lay on the ground without even trying to protect himself. Really bad acting/directing.