Unusual tactic?!

So I was playing a casual game of blitz, when I encountered this position:

Looks calm, right? After a Grand Prix attack formation along with Bb5+, Black is well developed, the Bishop is active, and White is just about to complete development.

And yet, because of the "just about to," there is a tactic for black here:

Cool, right? I don't think I've ever seen, let alone used, a tactic like this before. Have you? Tell me what you think!