Once upon a time there was an old castle with a beautiful thick oak door.
The thick oak door had two locks, one at the top, one at the bottom. Both had keys of shining brass, and whilst a little stiff, both locks fufilled their requirements adequately, if not perfectly. Both locks could be opened and closed from outside and inside.
One day it was decided to replace the bottom of the two locks, and replace it with a newer, shinier lock of brightest chrome. The locksmiths came and replaced the lock and tested that it locked and unlocked from the outside before declaring that their job was done, collecting their fees and walking home. Several weeks passed, and the servants came and went as they pleased, locking and unlocking the new shiny lock from the outside and commenting how much better it was than the old stiff brass lock.
Late one night, the prince was working late in one of the corners of the castle and the servants locked up before leaving for their hovels. When the prince was finished he found the majestic door locked. Unfazed, he produced his own key, an exact replica of the servants’ keys and tried to unlock the bottom lock.
To his shock and horror, he found that the key was too short to reach the lock from the inside. He tried pushing, pulling, twisting, turning, but the key was simply too short.
Suddenly a magical fire broke out in the kitchens, and the entire castle was engulfed in flames. The prince died and became a ghost, haunting engineers who don’t think comprehensive regression testing is important.