Here's a fun story that happened to my brother-in-law. Once upon a time, there was a playable demo on the Playstation 1 for a game called Lunar: Silver Star Story Complete. It was fairly lengthy, had a save game feature, and best of all, allowed you to import your save into the main game so you could continue the adventure from where you left off. The main character, Alex, carried an ocarina in his inventory. Its function? Doesn't have one, at least not in the demo. Not wanting it to take up valuable space that would be better suited for healing items, my bro moved it to the party inventory, where you can store excess items at the cost of not accessing them in battle. From there on, he forgot all about it and went on his merry way.
So he completed the demo, got the full game when it came out, and played through it. He laughed, he cried, he beat the [censored] out of that arrogant snot, Nash, blazed through the final dungeon, and reduced the final boss to a bloody smear on the floor. Then, he advanced forward to his final destination.
Now, here's the thing. There's one last puzzle to complete in Lunar: SSSC before the game can officially end; you need to play the ocarina before ascending the grand staircase. Otherwise, Alex will get zapped by lightning and die. In the full game, the ocarina cannot be moved at all; it always remains in Alex's inventory just so this scene can be done. This is its purpose, the reason why you carry it with you from the very beginning. But that wasn't the case in the demo. If you remove it from Alex in the demo and transfer that save into the full game, then you'll end up permanently gluing the instrument into the party inventory, where it can't be used.
So my brother-in-law had just reached the very last part of a 50-hour game, only to realize that there was absolutely no way in hell he would ever be able to win it. All because of some careless programming and a seemingly innocuous action done about 45 hours ago.