In the pool game scene Frankie sinks the orange stripe ball (the thirteen ball) three times on his run to sinking the eight ball. Of course this might be the rules of this one particular bar, but that's a stretch.
In the 1st shootout in Ireland, when Franky is looking out the window BEFORE the shooting starts there are already bullet marks in the wall around the window. Sure enough that's where the bullet's hit when the shootout commences
At the beginning, when Frankie's Mother is dishing out the stew for dinner, she sits down, picks up her napkin and then clasps her hands together for grace without serving herself any food. In the next wide shot there is food on her plate.
At the beginning, in Belfast it's supposed to be September 1992 but the tax disc on the silver car that the spare tyre is taken out of the boot has a tax disc that says "9 94" (September 1994) meaning it has to have a new tax disc at this date. These are valid for a maximum of 1 year so even if the disc was bought on that day the maximum date could only be "9 93" - a full year before.
The ring of keys that Frankie removes from the car's ignition does not have a handcuff key on it, and the two cops would not be able to pursue him in their car because the car is trapped in traffic, so there would be no point in Frankie's taking the keys with him, and he could not use them to open the handcuffs, as he later does on the train.
During the first shoot out Frankie yells to Ronnie and points to him. We see Ronnie turn the corner and start to shoot. However if you look closely debris is falling "upward". Obviously the shot is in reverse. The same shot is repeated moments later but correctly.
When Frankie shoots out the car's passenger-side window with the shotgun, a tight grouping of five or six bullet holes also simultaneously appears below the door's window frame. Obviously a pre-fabricated setup that was exposed by a small explosive charge and that was covered by the black paint on the door.