I was hanging out near a shrimping boat in the Corpus Christi marina, taking pictures of the great blue herons and brown pelicans that were staying around looking for handouts. When the shrimpers are around, the birds are fearless.
This gull was perched on a post nearby, and I was taking closeups of him with my 300mm Sigma. He kept cocking his head to the side, and there was a nice catchlight in his eye, so I was snapping pictures, and then he turned to face me and opened his mouth wide. The camera, luckily, was already focused and adjusted for the distance, and I was able to snap the shot before he closed his mouth again.
It was pure luck, really. I didn't do anything to provoke the guy.