Unexitable room bug
The unexitable room bug is a coding error, where the PC is able to enter a room but can't exit, even though there is no reason why the PC shouldn't be able to exit. In short, the author simply forgot to code the exit.
It is not a bug merely to entrap the PC in a room if the PC is meant to be imprisoned. It's a common trope to have aliens abduct our hero and cork him or her into a room with no exits. We're refering here to ordinary rooms that the player entered, through an ordinary door or doorway, only to discover that the author didn't code a way to go back to the previous location.
Some unexitable room bugs happen because the room is unimportant to the gameplay. If a closet or bathroom contains no puzzle pieces or plot tokens, a typical walkthrough of the game wouldn't use those rooms at all, and so the author might miss that those rooms can't be exited.
Some unexitable room bugs happen because the room is the final room of the game, and the author only tested the game's ending(s), knowing what inventory items to bring in advance. However, a player who reaches the final location without the right items will be stuck, because he or she won't be able to leave that room to retrieve them.