Yeah, I tried a REALLY shitty arm triangle attempt and basically wound up squashing the kid's face with my shoulder. He tapped, but it was his first day so I didn't think too much of it. I was still amped as hell to finally tap someone though.
Or, if you meant the first time I pulled off a GOOD submission.. I'd have to think about it, not sure I remember that one. Probably a keylock from side control, though. That has been my best submission by a long shot. It sucks to admit, but I've never submitted anyone from guard with anything besides a kimura.
One thing that does stand out was the first time I ever submitted a higher belt. It was no gi which is more my thing, but I tapped him out with a kesa gatame death squeeze similar to what Josh Barnett got Dean Lister with (it took a long time 20-30 seconds, but he tried to get my weight off his ribs and just couldn't and tapped), and he told me later on that he was a blue belt. I was like, "shiiiiiiiiiiit"