If it’s a Yale latch lock and it’s on the right-hand side of the door, then a loose snib might just have fallen down when the door was slammed shut. (The snib is the tiny button or lever that holds the bolt open or closed. A dropping snib on an ERA on the other-hand, is only going to lock you out if it’s on the left-hand side of the door. (These handings are as if you’re standing outside.)

Another possiblity for any lock with a snib is that there’s a huge gap between the door and the frame; and you left the lock snibbed yet managed to slam it shut.

One, more disturbing possibility you should bear in mind however, is that if a thief has broken in through a window or throught the back door, (s)he will usually snib the front door’s latch so as to get some warning if you return.

So, enter the premises cautiously. Send the dog in first.

If you’re choosing a latch lock, some can be snibbed open or closed, but some can only be snibbed open (also known as a hold-back).

This can be compounded if an inexperienced carpenter or locksmith has not secured the lock to the door with screws placed in the outermost fixing holes of the lock, but instead has used the innermost holes that ERA, also inexplicably, provide.

So if you recognize that you have this lock, once a month you should put a smear of heavy grease — car grease will do nicely — on the bevel of the lock’s bolt where it hits the keep’s strike. The lock will be on the door and the keep will be on the frame (unless your fitter was really incompetent). In the picture above, the bolt is at the left and the flat is facing us; the bevel is behind. The extra little dingus sticking out of the lock on the left is the anti-slip device that stops the commonest way of opening a latched front door without a key.