Revealing mistake: When Bean silences the guest in room 427 after they were playing loud music, he accidently locks himself out. Later when Bean crawls under a carpet past his room to hide from a kissing couple, room 427 has changed into a fire exit (you can see the room number has been replaced by a fire exit sign).
Deliberate mistake: Mr Bean pulls the shower curtain across the hole across the wall he has drilled (in order for him to have a bath in his 'neighbour's bath.) However in reality the shower curtain is on the wrong side of the bath. The point of a shower curtain is to stop the floor of the bathroom getting wet when showering. The only reason why the shower curtain was on the wrong side of the bath was for Bean to conveniently hide the hole that he drilled.