Every child reaches a point in his life where he asks a grown up, "Why did the Empire spend all that time and money building the Death Star, only to leave that vulnerable exhaust port wide open?" That question marks the end of your naive childhood and the beginning of your life as a cynical adult.
But Star Wars was more true to life than you think. History is full of unstoppable super-weapons and fortresses that tended to have one, ridiculous flaw the enemy could take advantage of.
The Helepolis was the largest siege engine ever constructed in the ancient world. It was essentially a huge, rolling fortress--about 13 stories tall and weighing around 180 tons. It was used by Demetrius I of Macedon in his siege of Rhodes, about 300 years before the birth of Christ.
The name Helepolis literally means "destroyer of cities," which is badass enough, but it was also being driven by Demetrius, who had the nickname "Poliorcetes," which meant "The Besieger" in ancient Greek. Either Rhodes was in deep shit, or this was a case of gross false advertising.
Behind each of those windows up there was a catapult capable of launching an object heavy enough to turn a man into a puddle of spilled spaghetti. In total, it had two catapults on the first floor capable of launching 180-pound projectiles (basically picture them throwing refrigerators at you) and one catapult launching 60-pound ammo. On the next floor up were three more 60-pound catapults, then the next five floors featured 10 catapults that could rain down hundreds of 30-pound projectiles until nobody on the ground felt much like fighting any more. Finally there was a pair of dart throwers on the roof to kill any defender on the walls of a city.
Oh, and it was covered in iron plates, making the structure fireproof.
The Fatal Flaw:
Building-sized vehicles tend not to be all-terrain.
If you're ever facing something that 1) is on wheels; and 2) weighs 360,000 pounds, you can bet on one thing: It's not going to be worth a shit in the mud.
The Rhodians made an educated guess about where the attack would come from, and before the rolling tower of badassery could get there, they channeled water and sewage from the city and turned the whole area into a bog of mud and human poop. When the Helepolis moved in for the kill, it got bogged down in the mire and a tower full of soldiers realized there probably weren't enough horses in the world to drag the their rolling fortress to dry land. They abandoned their superweapon.
History does offer an alternate, equally sad, version of the story where defenders were able to simply run up to the thing and pry off the iron plates, aka Luke Skywalker bringing down a walker by knocking a hole in it with his lightsaber. Everyone agrees the outcome was ridiculous either way.
Without the Helepolis, the siege of Rhodes failed. The so-called "Destroyer of Cities" was abandoned, having never destroyed a city. The Rhodians tore it down and used parts of it to build the Colossus at Rhodes, which was not in fact a big-ass superweapon, but just a huge statue of the Greek god Helios positioned so that every passing ship would be forced to look up at his junk.
4The Mary Rose
The Mary Rose, launched in 1511, was possibly the most badass warship the world had ever seen up to that point. It was in fact one of the first ships built solely for war--up until then, navies would just take merchant vessels and stick some guns on them. On the sides of the Mary Rose was a new invention: gun ports. Doors you could stick cannons out of. These allowed the Mary Rose to broadside other vessels and obliterate them. The ship had up to 91 heavy guns and several more anti-personnel guns, designed to take out an opposing ship's sailors.
The ship was ordered built by Henry VIII, motivated by one of the most powerful emotions known to man: hatred of the French. England was always threatened by the massive French Navy and the Mary Rose was intended to be the equalizer.
The Fatal Flaw:
It was a boat with big-ass holes in the side.
We're guessing you've never designed and built a boat before, let alone a warship, but you probably can guess the problem that comes from building a boat with lots and lots of holes in its hull. Those gun ports were placed really close to the water, and if they tried to turn (which causes the boat to tilt) without sealing up the ports, water can come rushing in.
And that is how the Mary Rose managed to sink without the enemy firing a single shot.
Recently named "The M. Night Shyamalan Effect."
It happened during the Battle of the Solent, fought between French and English fleets. If eyewitnesses at the time are to be believed, the Mary Rose fired all of her guns on one side, and went to turn around to aim the loaded cannons on the other side at the french. A sudden gust of wind caught the sails and tipped the boat over a little more, and much to the surprise of a whole bunch of British sailors, water came pouring in her starboard gun ports. Most of the ship was under water before anyone could figure out what the hell had just happened.
Now, there are accounts by the French claiming that their guns sank the ship, but why would the British invent a version of the story that makes them look stupider?
Out of the 400 men on board the Mary Rose, only 35 escaped the ship. Along with the crew, the pride of the Royal Navy was gone.