(Don't hit the) TREE! TREE!
I've noticed the same thing in (of all things) Frisbee golf. I'm trying to throw the disc around a tree trunk, and I'm thinking "don't hit the the tree, don't hit the tree", and of course I throw the disc and it hits the tree trunk, in spite of the small solid angle which it subtends.
I theorised that whilst I think I'm thinking "don't hit the the tree, don't hit the tree", I'm effectively shouting "TREE! TREE!" inside my head. And so I hit what I'm thinking about.
PS @ Ken Hagan: good point.