[DarrenX]

The subtelty is debatable, but I didn't catch it the first time or two I saw it:

Vanilla Sky, when David is yelling at Brian, blaming him for the accident involving Julie Giani, because Brian must have told her that David considered her a "f*ck buddy". Brian immediately denies this, saying, "I told her nothing of the sort", followed by his eyes quickly darting quickly from side to side- I took this as either:

1) him blatantly lying and his eyes giving him away or

2) him telling what he believed to be the truth, then 'searching his memories' of the drunken night of the party, realizing he actually may have said it.

Either way, it seemed blatantly obvious later, but perhaps not so to the casual observer.