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."
1. The blood drop scene in The Incredible Hulk took a year to get right
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.