Wars have been won and lost purely on the ability of one side to hide really well. Even a superior enemy can't take you out if they can't find you. Give ancient Rome barbarian-seeking arrows, and we'd still be wearing sandals to the bath house to wipe ourselves with sticks.
OK, bad example.
Finally, in the 21st century, technology is catching up to the problem with weapons that almost seem like cheating. So we're not sure if we should be impressed or terrified by...
6Bullets That Will Outright Chase Your Ass Down
Really the only thing there is to know about bullets is that they travel in a straight line. If a dude is shooting at you, you're fine as long as there's something good and solid between you and him. A car door, a sandbag, another less fortunate person you're using like a bullet proof vest. This has been the rule for the last 700 years or so.
Until somebody came up with a way to develop guided bullets.
The building full of crazed geniuses known as the Defense Advanced Research Projects Agency (DARPA), have developed the EXACTO program, which involves using "smart" bullets complete with processors and steering vanes to allow the bullet to correct its trajectory in mid-fucking-air.
So even if you duck around a corner as some asshole is shooting at you, his bullets will supposedly be able to just swing around and follow your ass. We aren't talking about some wimpy small-caliber bullets either; the only size they are building these in (for now) are .50 caliber BMG, huge rounds which are intended either to kill trucks, or explode a guy's head from several miles away.
Just in case the word "overkill" hasn't entered your mind yet, they've got a much more explodey version of that idea, too. It's the XM-25 "smart" grenade launcher. It has a laser range-finder built in that can detect the distance to the wall or trench that you're hiding behind. That way the shell knows not to explode until it's right next to your fucking head. That's right; it won't detonate until after it punches through the wall you're hiding behind, but just before it hits you in the nose.
The XM-25 has a range of different munitions it can use, from high explosive to thermobaric, all of which are designed to make your trip to the other side as sudden and unexpected as it is horrifying.