Deliberate mistake: The priest that Ray kills doesn't stagger out of the confessional booth toward the little boy until he's already been shot twice, yet when Ray sees the boy he accidentally shot, he is still kneeling in the prayer position and therefore did not move or react whatsoever to the first few gunshots only a few feet away.
In Bruges (2008)
1 deliberate mistake - chronological order
Starring: Ralph Fiennes, Colin Farrell, Brendan Gleeson, Elizabeth Berrington
Revealing mistake: During the final shooting, a bullet hits a lamp post, causing a spark. A grey tape glued over the original metal surface is noticeable to help produce the spark.
Ray: Why didn't you wave hello to me today when I waved hello to you today?
Jimmy: I was on a very strong horse tranquilizer today; Wasn't waving hello to anybody. Except... Maybe to a horse.