Revealing mistake: When Gerard Butler is driving to the warehouse to find Frank, the speedometer and the rev counter are on 0.
Suggested correction: I could not see Will's speedometer or tachometer (RPM gauge) during his drive to Frank's. What I did see (and believe you saw) was the patrol officer's instrument panel while he was parked along the road using radar. This was before he pulled out to chase Will's speeding SUV, so both gauges should have been on the far left (i.e, on "0"). [00:46:28].
Revealing mistake: When Ritchie tackles Sam down the stairs, you can get a pretty clear look at both the stunt-man and stunt-woman's faces as they fall. They then switch back to the actual actors once they reach the bottom of the stairs.
Revealing mistake: The owl that JP saw was obviously "fake" (stuffed or CGI). Its nose was crooked and its puffy cheeks made it look like it had cheek filler injections; owls have flat faces. (00:32:50)
Revealing mistake: Max rolled down the front passenger side window and no wind came into the car, but when he had his face barely out the window, his hat blew off (not shown on screen). If wind hit the visor part of his "bugout" hat, the hat would have most likely blown off into the car. (00:03:40)
Revealing mistake: After the accident, some bystanders rush to the car hit by Diana's. A woman leans against the car window spotting a child inside; you can see on the window very conspicuous handprints in that spot from previous takes. (00:17:10)
Revealing mistake: The cell bars appear to be about 4" apart, but are much wider to accommodate the width of The Artist's face/head when there's a close-up of him next to the bars. (01:13:50 - 01:15:09)
Revealing mistake: Luke points a flashlight along the side of the staircase, but no shadows are produced on the wall from the vertical posts. Luke points the flashlight in front of himself and produces his shadow on the wall behind (and the shadow doesn't look like the way he is holding the flashlight). After Luke goes through the entryway aiming his flashlight ahead, a circle of light is behind him. The power is out, so the flashlight is the only light source - except that used for filming in the dark. (01:03:00 - 01:05:25)