There are actually two film versions of this novella. One is from 1939 and the other from 1992. I'll assume you are referring to 1992.
Candy never came into the room when Lennie and Crooks were talking to each other.
Lennie's hallucination...
The Question and Answer section for Of Mice and Men is a great resource to ask questions, find answers, and discuss the novel.
There are actually two film versions of this novella. One is from 1939 and the other from 1992. I'll assume you are referring to 1992.
Candy never came into the room when Lennie and Crooks were talking to each other.
Lennie's hallucination...
Candy means that Curley was good at boxing in his weight class.
Unless George watched over Lennie at all times, I don't think something like this could be prevented. Lennie would inevitably make this type of mistake because he understood very little about his own impulses and others didn't understand him.