If you're describing the look on Mr. A's face, perhaps he had an ashamed, stunned, or embarrassed look. It may have lasted only a moment until his neo-cortex kicked in and suppressed his limbic response to the difference between what he told you and what he just said to Mr. B.
If you are describing the moment of realization, there is a neologism, ignosecond, which Urban Dictionary describes as
The moment of clarity just after a boneheaded act, but just before the point of no return, when you realize you've just done something stupid
Other ways of describing the realization might be
- Mr. A. realized that I had caught him in a lie.
- Mr. A. realized that he had spoken a bald-faced lie.
- Mr. A. was hoist by his own petard. (Or "Hoist with his own petar" from Hamlet Act 3, Scene 4)
- Ka-ching! Whoop! Whoop!
Pick your point of view and level of formality.