2. Irrigation - Idiocracy
Unlike all the other films on this list, which have inventions being made prematurely thanks to time travellers, the underrated satire Idiocracy features irrigation, which actually has to be reinvented because the human race has devolved to the point where we've forgotten one of the most basic farming techniques. Seriously, the Incas had a grasp on this, and they practiced ritual human sacrifice. In the future, all the stupid people have Duggar-sized families, while the more educated are sensible and have historically lower numbers of spawn, leading to an intelligence level that has gradually decreased over the span of several hundred years. These people have basically been trying to feed energy drinks to plants, and are somehow shocked when nothing grows. Until a visitor from the past appears and suggest that they give water a try. Water? From the toilet?! Unsurprisingly, this has an immediate effect on the crops, and the future of this ridiculously depressing society is ensured. Although really, we shouldn't be so hard on them -- Bad Grandpa was recently nominated for an Academy Award, so we're probably looking at Idiocracy in about twenty years or so.