When Hellboy goes down in the sewers to search for a missing Agent that they fear is killed, his hands are completely switched with the huge stone hand being his left hand and his gun being his right. It lasts about 5 seconds
This is an outstanding slip-up but not quite for the reason posted. After thinking, how could this happen? Why would a right-handed fist even exist? Well if you study Hellboy's costume and the table he passes [this occures at about 59 minutes into the movie] you'll notice a buckle on Hellboy's collar and a patch on the sleeve of his "gun hand" as well as everything on the table is backwards [note the chisel set from 54:40]. Somehow the entire clip as been horizontally fliped! Still, a major Slip-Up!