Dangerous Sixth File
There are two diagonals which should be protected especially at the opening: the e1-h4 for White and e8-h5 for Black. Pushing the f pawn prematurely opens this diagonal, putting the king in peril. Lesson: don't push those pawns! This also focuses the problem of many players who are pawn-pushers, as if there are only pawns on the board. Pawn formation should be in one's mind in the opening too.
Any evidence of this? See the diagram.