Deliberate mistake: When they are bringing the Cook's body into the study, it's obvious the Cook is actually the Cook's double because she is much thinner and takes up less space in the doorway. (00:34:40)
Deliberate mistake: When the chandelier smashes after Colonel Mustard and Miss Scarlet are freed from the lounge and at the end of the first ending, the light level doesn't even go down. (00:53:15 - 01:20:30)
Deliberate mistake: When Mrs. White is screaming after the power cut, she then says "Help me!" multiple times. It's obvious the "Help me!" lines are just a recording played over and over. (01:03:15)
Deliberate mistake: The same footage of Ms. Scarlet screaming when the cook falls out of the freezer is used when Wadsworth falls out of the freezer. (Note the cigarette.)
Deliberate mistake: Wadsworth was explaining how Miss Scarlet killed the cop. It then showed her killing him. You can see that she stopped the lead pipe behind his head so it wouldn't show a violent and bloody death.
Answer: Yes, the accent was fake. She wasn't a French maid, she was a prostitute who used to work for Miss Scarlett. She was hired by Mr. Boddy.