It's doorways. Seriously.
Your brain uses a very similar directory system to that of your computer. Only instead of neat folders labeled "Work," "Documents" and "TOTALLY NOT PORN," your brain tends to compartmentalize by physical location. This means that the information readily accessible to you in one room ("I must get a glass of milk to wash down all this delicious fudge") suddenly becomes a lot harder to access when you go to another one ("Why am I in the kitchen? I know it had something to do with the toaster ..."). The moment you cross a doorway, you're essentially sending a signal to your brain that you're in a new environment now and that nothing that happened in that previous one matters, so just flush it.
Radvansky tested this by having students examine a box containing objects such as red cubes and blue spheres. Then, the students tried to remember what those objects were after either walking into another room or just walking that same distance without crossing any doorways. The results were so dramatic that researchers proceeded to redub doorways "event erasers," a name so badass that it is what we're going to call doorways from now on.
"Son? I'm sorry, but I never had a son."
And the effect of doorways is so strong that you don't even have to physically move for those bastards to put the kibosh on your memory. In another experiment, the researchers had people sit at a computer and do the same test, where the new "room" was just an animation on the screen. The effect was exactly the same -- every time their avatar crossed a virtual doorway, their ability to recall objects fell down the forgetting well.