In this era of massive budget blockbusters and even bigger "shared universe" movie franchises, it's safe to say that we're not always looking for realism at the cinema. While films are capable of conveying lots of different sorts of messages, the common thread that binds them is entertainment, and as such, reality often falls to the wayside in favor of plot convenience, storytelling, or sometimes, just a lack of scientific understanding.
Movies that are "based on a true story" tend to bear little resemblance to the "true stories" they're based on, movies about the military almost invariably fail to capture the culture or even the vernacular of American troops, and the Fast and Furious franchise has a physics all its own… but some movies do a good job of establishing that the rules of their cinematic universes are similar to our own, only to offer up weapons that, at best, don't make sense, and at worst, would leave their user reduced to little more than a puddle of goo.
Some of these nonsensical weapons play small roles in the movies they inhabit, while others, like these, have become cultural touchstones; serving as symbols of the fictional universes they inhabit and the fandoms they inspire. These weapons are cool, dynamic, exciting… and would totally get you killed in a real fight.
1.The Klingon Bat’leth
While the Klingons had already been around for some time before "Star Trek: The Next Generation" introduced the Bat'leth, the unique double-sided sword quickly became visually synonymous with the Empire of warrior aliens. There's just one problem: melee weapons make no sense in a galaxy full of handheld phasers and disruptors, and even if they did -- the Bat'leth is one useless melee weapon.
While most bladed weapons offer the user an increase in reach, the Bat'leth's curved shape makes it more awkward for extended one-handed strikes like a bow or staff might allow, and while held in the traditional two-handed way, it offers little more than a solid defense against other melee weapons. Perhaps this is why the mighty Klingons always find themselves bested in hand to hand combat by humans, Bajorans, and anybody else the plot finds convenient, despite their fierce reputations.
2.The Jedi/Sith Lightsaber
This one is sure to ruffle feathers, as the Star Wars fandom has devoted a great deal of time and energy to explaining away how these energy weapons must really work. However, as of Disney's purchase of the franchise, canonical sources have been slashed, and we're left once again with lightsabers that work without the plot-hole filler that was once allotted.
What we're left with are extremely hot energy weapons that, as others have pointed out, shouldn't work because the beams have endpoints, but assuming they did -- anything that could burn so easily through feet of steel as depicted in the films would also melt the meat off of your hands as you held it. It would take so much heat to do what lightsabers are depicted as doing, it wouldn't be safe to be in the same room as one, let alone to start swinging it like a baseball bat.
3.Tony Stark’s Iron Man Suit
The Iron Man suit has become one of the most recognizable symbols of the Marvel Cinematic Universe, and with good reason. The MCU as we know it was born with the first Iron Man movie and in many ways, Stark serves as the Skywalker of the series… but that doesn't change the fact that the suit that grants him his powers would actually be his undoing.
While the Iron Man armor may protect Tony from impacts and penetration, it can't stop inertia. Iron Man is regularly shown taking hard, nearly instant turns at jet-fighter like speeds and even hitting the ground at similar velocities (whether intentionally or otherwise). Even if the armor offered protection from impact, the inertia of those movements would turn Tony Stark into chunky stew.
In reality, the first Iron Man movie likely would have ended with Pepper Potts prying the suit open only to let what was left of the titular hero pour out… which is why maybe it's not always good to be completely realistic with one's movie weapons.