When you're checking out a hot girl and she was totally expecting it, so you quickly look away, but at this point its too late.. cos its blindingly obvious you were caught in the perve.
It doesn't always have to be the girl that catches you, sometimes it can be the boyfriend of the girl who you only notice a second later, when hes giving the evils.
dude: mate, check out the girl over there!
mate: o yeah awesome!!..o crap...
mate: I just got caught in the perve
innocent dude: woah!! that girl has nice tits!
boyfriend: what the f*** you think you're doing checking out my girl!?
innocent dude: crap