Crook? or Liar? (link)
This is what the internet is about.
It's fine to be stumped on a question, but not okay to pause in order to figure out what other lie you have to not untell with your next utterance, When the world can see the calculus of dishonesty cross your face, you're a gonner.
It's the same look when an actor momentarily forgets his line, or panics for a second when forced to ad-lib. Momentarily out-of-the groove.
it's a tell.