I've also thought it would be funny to see a picture someone posted on the boards from their trip and then see myself in the background. So far that hasn't happened, but I always look.
I have totally done this! I have taken a fair number of solo trips to WDW and I manage to have conversations with many strangers and have pictures with them. But outside of WDW, this happens a fair amount with my group of friends. We all tend to be rather outgoing and have many pictures taken with strangers. Mostly this tends to happen for me while a bit ... umm ... tipsy. My favorite one is below. I have no idea who he is but he seemed very fun with that devilish, sarcastic grin
No, but I did once ask a couple at MNSSHP if I could take their pic because their costumes were the best I'd ever seen. They agreed without hesitation.
It was just a few minutes to closing on a MNSSHP a few years ago and my family and I made a run (literally) for the Haunted Mansion! We barely made it in and as the doors closed there were some the "ghosts" just like from the ride with us! They were SOOO cool that we had to ask to have their picture with my daughters! I think they enjoyed it too!
Some friends of mine travelled to Japan on business and took their daughter with them. Their little girl has bright red, curly hair and the amount of people wanting their photograph taken with her was unbelievable! To them her hair was a real novelty. She even picked up the 'peace sign' a lot Japanese tourists like to pose with and they all thought it was a scream! After a while her parents had to start saying no, because they couldn't get anywhere.