I'm still feeling a bit emotional about it too!
I took DCs to Morrisons cafe as we needed a couple of bits on the way home from the park and they were getting hungry. I went to pay and my contactless card wasn't working 
I couldn't remember my PIN either and after a couple of tries I was getting really flustered and hoping the ground would swallow me up. Typically I had left my credit card at home and no cash on me. I went to get DS who was sitting down at a nearby table to leave (bracing myself for the inevitable tantrum, understandably!) and the gentleman behind me in the queue offered to pay for our lunch!
I've put something on my FB page and hoping to track him down so I can repay him and I will certainly do the same if I see anyone else in that situation (provided I can remember my sodding PIN). 