American Football (and most other sports) constantly changes its rules, either to address shortcomings, adjust for changing technologies, or make the game more commercial.
About ten years ago I heard of some rules changes they had made to make football more exciting for the fans to watch. Apparently the classic running game where burly offensive linemen shove back burly defensive linemen and the fullback makes a consistent 2-3 yards isn't very entertaining. So the powers that be decided to make the rules more passing-friendly.
I don't know all the details but I think one of them (at the time) was the no-bumping rule. It used to be that the defender (a cornerback usually) could "bump" and shove around a little the wide receiver or whoever was going out for a pass. The rules change made it so that you couldn't bump the receiver quite as often, or as early, or something like that. Some other various rules were thrown in as well to make the passing game a little easier.
Seems like the right approach -- you want to promote passing so you make passing easier.
However, the upshot was a bit different from what they expected. You see, coaches hate big exciting passing plays ruling the game. If you're in charge you want consistency, not randomness -- and these guys are already nursing enough ulcers as is. So, since making passing easier makes offense easier, the coaches shifted more and more of their best players to defense.
Also, defensively it was no longer efficient to defend against the pass by covering the receiver -- the no-bumping rule makes it too easy to accidentally foul, or simply for the receiver to escape. However, it's still plenty okay to hit the quarterback, as long as he's still holding the football. So the blitz is on -- rush the biggest, fastest, and strongest players on your team (some of whom may have once been offensive linemen) straight at the quarterback and hit him as hard as possible before he releases the ball. Sure, you'll give up some big passing plays in the early part of the game, but toward the second half the quarterback won't be so mobile, might be a little dizzy, and might not even be playing at all.
The lesson for me was fascinating and valuable: rules that might otherwise seem intuitive can have consequences entirely contrary to your goals. After all, the last thing the NFL wanted to do was put all its superstar quarterbacks in the hospital; that would certainly kill the passing game.