Been called out to two lockouts in the last two days where the customer had been locked out due to the snib on their nightlatch dropping down and preventing the key withdrawing the latch. The snib had dropped down when the door had been pulled closed. This happens when the mechanism becomes worn over time. This is quite a common reason for locksmiths to be called out. You can prevent it happening by ensuring the snib on your lock has to be moved upwards to engage it. Different makes of lock go in different directions so choose one that needs to be moved up when facing the way it will go on your door. If in doubt ask a local locksmith for advice.