Take the 23 enigma, the theory that all events are somehow connected to the number 23. A load of mystical clap-trap, of course, but I defy anyone to watch the Jim Carrey film The Number 23 and not be spooked afterwards when they start seeing combinations of twos and threes everywhere they look.
What's happening is known as confirmation bias - when you believe in something, it's easy to find evidence to back it up. Learn a new word or discover a new TV show and, suddenly, you can't get away from it.
The flipside is that we often fail to see what's in front of our nose, because we're not looking for it. Ask a colleague to wander around the office counting red items. They could tot up 50. Ask how many yellow items they saw and they may struggle to remember more than five.