( For some reason it wouldn't let me select Improvement instead of Bug )

If you have a 3+ block high wall and place a 2 block high glass or iron bar window in it so you get a good view of the outside, because the pane/bars are thin and in the center of the block, mobs can jump on top of the solid wall block below the pane/bars, and smoosh their face against the window and make funny faces.

It would be nice if placing them worked the way doors do; if you click on the edge of the block below, then it gets aligned to that edge instead of being centered. That way you could make sure the window presented a smooth face to the mobs outside.