I just think about coincidences in terms of brain mechanics... that thinking about something 'primes' it in the neural networks of the brain and biases input making it more likely that you will notice things related to it... things you would not notice if you hadn't been thinking about it. So if someone notices something and says 'that's funny, I was just thinking about that the other day' then all I can really say is 'there's your answer'
