There are thousands of awesome movie moments out there that are a result of a last minute change or improvisation. This is not a list of those things - this is a list of moments from movies that although pretty cool, took so long to film or include you kind of wonder why they didn't just say, "Eh, screw it, lets think of something else."
As fantastic as that shot of a single drop of Bruce Banner's, radiation-filled vein gravy bobbing and weaving through a random factory in Brazil was, I'm pretty sure the movie would have still gotten the overwhelmingly average response it ended up receiving if it wasn't included. It may have even gotten a better response because without it we'd have seen the Hulk punch something like 10 seconds sooner.
For some reason though, the company behind that shot, Image Engine, spent upwards of a year tweaking that scene to get it just right. Now I'm no expert on making movies, but I'm going to go out on a limb and suggest that if Image engine had spent the budget and time that, that scene ended up consuming on something else, like maybe the Hulk kicking Abomination in the sack, the audience would have appreciated it more than an artistic shot of a drop of blood in freefall.
Sources: A Sci-Fi Movie Lexicon II / The Wire
Fun fact, every frame in Jurassic Park featuring a CGI dinosaur took about 2 hours to render. If you do the math it means that just 5 seconds of sweet, sweet dinosaur action took about 10 days of real time to render. Which is fine, because Jurassic Park is probably the cure for eye cancer in movie form, that movie is so radical if you set copies of the it on fire it sounds like a 50's surf rock album.
However, literally days were added to the time it took to render the movie's most famous scene by one thing, the inclusion of rain. In fact, rendering the movie's iconic T-Rex took upwards of 6 hours, per frame just because the scene in which it appeared happened to take place when it was raining. If that wasn't bad enough, including rain in that scene also messed things up on the animatronic side too because the big-ass foam dinosaur had a habit of soaking up all the water and shaking at random, scaring people on set. Which although hilarious, makes you wonder why they didn't just decide to set the scene at night when it was dry to make things easier. It's not like the T-Rex snuck up on them or anything, they knew it was coming from a mile away because of the ripples in the water, water that was being kept inside a glass INSIDE OF THE CAR. That was literally the only place in the scene that they needed water and it probably took a runner like 30 seconds to include it. The weeks worth of rendering and all those giant dinosaur sneezes were just unnecessary.
According to Sylvester Stallone, the scene in which Rocky pounds on a side of beef like it owes him a months rent is supposed to represent the boxer coming to regard his opponent as nothing more than a "nameless, meaningless" piece of meat. According to everyone else who saw that movie, it was to show that Rocky could punch really hard. Regardless of how that scene is interpreted, it's hard to imagine that it took that long to film. I mean really, how much footage would you need of a man punching meat?
Well, according to the official scrapbook of the movie, compiled by Stallone himself, the crew spent 14 hours in that freezer uppercutting the shit out of a bunch of steaks. Stallone actually got so into the role that he ended up punching one of his own knuckles completely flat by hitting the meat too hard. If you break your own hand doing something, it's probably a sign that you've been doing that thing for too long.