1) When you are watching something so intensely that you comletely ignore all hazards or obstacles around you and collide with one of them
2) When you are watching something you are trying to avoid so intensely that you forget that you are trying to avoid it, you accidentaly collide with it.
Happens a lot in video games.
Person A: Dude, at break I was trying to avoid getting scolded by Mr. Miller, but I was so busy looking at him I realized I was walking straight into him.
Person B: Haha, thats target fixation!