Same here! (NW 2731 on Feb. 22, 2009, to be precise - my last NW flight as they were integrated with DL shortly thereafter).

We were late inbound, but I got to the gate and saw that boarding hadn’t yet begun, and decided I had time to go to the restroom. I returned barely 3 minutes later and found the gate area empty, and the agent saying he was just about to close the door - I’m usually on mainline, so I guess it didn’t occur to me it doesn’t take long to board 7 passengers.