There are cases when (due to actions being taken in the 'real world' and not being recorded in OpenERP) quantity of stock in the system doesn't match quantity of stock in the real world.

Because of this, OpenERP permits the user to move stock (and sell things) that it doesn't have a record of.

1) It would be very frustrating if someone saw 10 units in location A, moved them, and OpenERP refused to let them record the move (which actually happened in the real world.)

2) It would be a bad implementation if OpenERP refused to let someone sell something which actually existed in the real world, just because someone didn't process a picking yet.

While you can certainly modify the software to prevent this from happening, there are implications that you may not want - this is why it works this way out of the box.