Well, according to this article in Salon (originally an article in Scientific American), it's not just poor attention or early-onset Alzheimer's; it's a real thing.
Gabriel Radvansky, Sabine Krawietz and Andrea Tamplin seated participants in front of a computer screen running a video game in which they could move around using the arrow keys. In the game, they would walk up to a table with a colored geometric solid sitting on it. Their task was to pick up the object and take it to another table, where they would put the object down and pick up a new one. Whichever object they were currently carrying was invisible to them, as if it were in a virtual backpack.
Sometimes, to get to the next object the participant simply walked across the room. Other times, they had to walk the same distance, but through a door into a new room. From time to time, the researchers gave them a pop quiz, asking which object was currently in their backpack. The quiz was timed so that when they walked through a doorway, they were tested right afterwards. As the title said, walking through doorways caused forgetting: Their responses were both slower and less accurate when they'd walked through a doorway into a new room than when they'd walked the same distance within the same room.
Further along in the article, it is contended that the act of walking through the door changes the way we evaluate the importance of what we're trying to remember. It wasn't just the size of the monitor, or even the new room that changed memory (read the article to see how they controlled for that stuff).
Why did I start this post?