I have printed over half a million books, and over 200,000 comic books. Three times over the years the printer made a coloring error on my job, and they agreed to destroy the entire print run each time (over coloring!) and re-do it right, and they did (absorbing all the expense, because it was their error each time). That is one reason they are happy to run lots of proofs, to make 100% certain the job is done right (and that everyone involved has signed off on it), so that they won't have to reprint.
In movie posters, there are lots of people who have an interest in the job that few others would care about. A minor actor might have been promised a certain billing, or the music credit might be contractually agreed to be done a certain way, and it may seem like no big deal to YOU, but it is to the people involved, and if a mistake is made that means scrapping the print run and starting over.
That brings up another point. In the three cases I mention above, I trust the printers involved DID do as they say, because I was dealing with really good people, but there are tons of sleazy printers out there. If you don't know it, many of the movie posters that end up in dealers' hands were "over-runs" from the printer that they do without studio permission. That is one reason Fox and others go crazy over auctions for their posters, because they never got any money for them in the first place.
Also, in my case, one of the three times I asked them to destroy them all, and I believe they did. One time I asked them to sell me the "bad ones" at a cheap price, and I sent them to stores as free samples. In the third case I asked them to give me 25 copies with the "wrong" color, and they did, but for the life of me I have no idea where they are (it was the very first EC reprint I did in 1972, and they no doubt would now be rare million dollar comics!).
Maybe they never did destroy them, and someday after I am gone someone will surface with rare Bruce Hershenson "printer proofs" of my books!
Bruce