When surfing pornography and seeing a video you recognize from years before and begin furiously beating it in response.
Dude, I saw this video last night that I hadn't seen in years... totally deja fapped it...
Magic sense to know when you're about to get caught having a wank.
Be alert, gentlemen.
Man, I almost got caught fapping by my mom, but then deja fap kicked in, and I covered up just in time.