As an adult male in what situation did you have an unwanted penile erection?

I do not find my sister in law attractive but was chatting with her in the living room while my wife went to the kitchen to make some tea. She sat opposite and I caught a glimpse of her knickers as she moved - shamefully I felt the unwarranted erection begin. As she chatted away blissfully ignorant of what I was desperately trying to hide my wife came in with a tray of tea and biccies and asked me to clear the coffee table, which would have meant me standing - in more ways than one. I managed to pull my shirt out hiding the offending state and feigned stomach pain to dash to the bathroom. Uncontrollable nightmare, can you recall your organ demonstrating the "mind of it's own" syndrome?