I am sorry to say that people has discovered a couple if errors in this brilliant movie. It must be a person who isn't doing anything else that to study movies and look for errors in the script. Or.....
Anyway, as the goofs are a part of movie, and maybe shows that errare humanum est - it is human to make errors, here they are:

Continuity

In one of the solutions, Miss White strangles Yvette by waiting for her in the billiard room. In the scene before this she is still heard screaming upstairs as Yvette enters the room.

Plot holes

When the characters run to Yvette when she is heard screaming in the billiard room, she says that she is frightened as she has also drank the cognac. However, in one of the solutions, she is killing the cook in the kitchen when the rest of the characters were making comments on the cognac being poisoned. Yvette would not have know about the poisoned cognac.

Grand error

When the murderer pulls the lever that cuts off all the power, the scene goes directly to the cop saying, "Hello? Hello?", implying that the telephone line has been cut off. Yet when the murder later goes into the library to kill the cop with the lead pipe, the cop is talking again, saying "There's somthing funny going on around here, and I don't know what it is, but I have the feeling that I'm in danger. No I'm not on duty...", implying that the phone line is still intact.
Noel

Grand error II

When the power is cut off, the phone lines wont go down.
Tom Reddish

A thought to that reported blooper:
The cop might not lose the connection, he may just say "Hello?" as if the telephoneconnection was cut off at the same time as the power. This is proved a couple of scenes later when he actually continues his conversation, and the power is still off.
Laurie Stark

Another thought to that reported blooper:
In the era in which Clue is set, telephone lines were directly routed through the electric power lines - thus, an electric power cut off would disable a telephone.
Jet

Visible microfone

When Wadsworths begins his first tour of "let-me-tell-you-what-happened", and he stands at the front door saying that Col. Mustard was the first guest a microfone is clearly visible in the lower right corner.