Some games with completion rates sometimes block the player from getting 100% (or more) when they cross a certain part of the game. If you want to get the 100% completion award, you have to replay the whole game in a different way.
- Some games completely lock access to certain areas after some events occurred.
- Some games will 'lock the door' after you went through it, or have you jump down a cliff which is too steep to go back up, effectively preventing you from going back to where you came from.
- Some games will require you to do some actions in a certain order to have a bonus or an award; the order in which you perform the actions is not important to reach the end of the game.
Some of these "barriers" seem to be completely "artificial", and are not told to the player beforehand.
These barriers are completely "artificial" and only seem there to piss off the player: no warning, absolutely no reason for why you can't go back, you just have to deal with it and replay the whole game if you want to really complete it.
Why would a developer want to do this? What's the point in forcing players to replay the whole game again if they want to 100% it?