We went to a customer today who was locked out because the snib on their night latch wouldn't let them open their door with their key. Usually this happens because the snib drops down when the door is shut due to the lock being worn. However this snib had to be pushed up to lock the latch. The door should not have been able to close in this situation. The reason the door had closed with the snib engaged was because the keep wan't firmly attached to the door frame and had flexed so much when the door was closed that the latch had pushed it aside and then engaged in the keep once it had sprang back into position. As you can see sometimes it works out cheaper to have your lock fitted by a locksmith rather than have to pay emergency call out fees when it goes wrong and you are locked out of your own home.