Some years ago a customer gave me a key to their back gate, and one day I mistakenly used it to open another customer's gate. I've since tried it in many of the same type of lock and discovered that it opens more than 50% of them. It's very handy as it means I don't need lots of keys for that type of lock. I must say that I do tell them that I have it and only use it with their permission, and I have had one or two change the lock for a more secure one and given me a key anyway which amuses me even though I understand their reason for doing so.