I've had two such experiences in my hobby life.
When I was in college I tended bar in a club down in PA. After graduation I came back up north to start my career and hadn't been back to the area for 20yrs.
Some buddies talked me into going to a 20th reunion gathering, so knowing I was going to be there for a few days I did some research and made an appointment with a highly rated young local gal. The night we got together was a fantastic encounter and we hit it off swimmingly! She actually ended up staying the night. In the morning I invited her out to breakfast and we really got to talking. During the conversation I mentioned why I was in town and the fact that I had worked at a club nearby while I was attending school there. Well right away I knew that something was wrong just by the look on her face and her sudden mood change. It took a little to drag it out of her but she finally admitted to me she was the daughter of the owner who I had worked for. DOH! I remember when she was born, but she was all of 4yrs old when I'd last seen her. Needless to say I went through a range of emotions on my 5hr ride home that day.
The other occasion involved an UTR woman I meet and saw for a few years. She had a select few she entertained and I was lucky enough to be one of them. She was attending Grad School and this was a way to keep up with expenses and, as she was completely upfront about,"not having the burdened of all the emotions that came with a real relationship". It was an almost perfect arrangement and we never really spoke too much about our private lives. For the better part of 3yrs the sparks flew anytime we got together. After she finished her degree we kinda drifted apart as her availabilty became very sporadic.
That summer my SO made a job change and 6 mo later we attended the company Christmas party. Well we weren't in the room two minutes before I spotted her. My first reaction was to panic, but I held it together. A few glances across the room and I was sure she had seen me, but she was as cool as a cucumber so my intial anxiety faded. But my real test was about to come when my SO said "hey stay here a minute I want to introduce you to my boss". Well I damn near shit myself when she walked across the room and grabbed my lady friend and started heading back my way! Hundreds of thoughts, explainations, and excuses raced through my head in the :30 seconds it took them to reach me. While my SO was giving the intro I had no idea what was going to happen or what I was going to say, but once again a calm and enthusastic reaction from the "boss" saved the day!
Later in the evening we finally got a chance to speak with no one else around. She admitted to me she knew I was going to be there and that her new employees was my SO. She claimed she had the exact same feeling of panic as I did but it was months before when she saw my picture on the desk at work.
Afterwards I was kinda pissed at the fact that she didn't try and contact me to give me a heads up instead of taking the chance that I may have said or done something dumb as an initial reaction. I got over it quickly and banged her one last time for New Years. It was by far the hottest encounter we ever had!