It's kind of scary the things we see but don't see. Recent studies have shown that we have a remarkable inability to retain images in our short term memory. A couple of experiments that demonstrate this:

In one experiment, people who were walking across a college campus were asked by a stranger for directions. During the resulting chat, two men carrying a wooden door passed between the stranger and the subjects. After the door went by, the subjects were asked if they had noticed anything change.

Half of those tested failed to notice that, as the door passed by, the stranger had been substituted with a man who was of different height, of different build and who sounded different. He was also wearing different clothes.

In another experiment conducted by scientists at Harvard:

They played the tape [a videotape of a handful of people playing basketball] to subjects and asked them to count the passes made by one of the teams.

Around half failed to spot a woman dressed in a gorilla suit who walked slowly across the scene for nine seconds, even though this hairy interloper had passed between the players and stopped to face the camera and thump her chest.

More about the studies and what they reveal about how our mind processes images here.