What's your favorite continuity joke?

Hands down, for me, it's everyone eating "sandwiches" in college flashbacks. I imagine if you missed the one reference they made to what it actually was, you'd be incredibly confused. It's also a hilariously brilliant way to get around censors.

What about you? A goat on Ted's 30th (and 31st) birthday? Ducky tie for a few shows? Robin Daggers, née Sparkles?