Did you notice that this picture shows a house key stuck into a Ford ignition lock? When this happens never try to pull on the key with any level of force. It will break off creating a worse situation and possibly damage the lock, necessitating replacement. Puting the wrong key into a lock happens quite frequently. If you call a professional locksmith immediately the key may be extracted without damage. If the wrong key was forced into the lock with extreme force, the lock may have to be replaced. The lock in the picture had the key extracted by a professional and the lock was not damaged.
Many times the correct key is broken off in the lock. If the key is sticking out of the lock, you may use pliers or forceps to pull it out. Never push the key in further. If the broken piece is not sticking out of the lock, do yourself a favor and call a professional locksmith to extract it. Sticking tools into a lock can cause damage and in most cases where someone who did not know what they were doing tried to extract the key, the lock was damaged and needed repair.
Never and I repeat never use superglue or some other adhesive to try to remove the key. I am amazed by the amount of people who try this. It always destroys the lock. Key extraction requires special tools and a very good knowledge of lock design.