Some of you may not realize that there’s a very complicated and almost universally disliked system for assigning screenwriting credit on a movie. The process itself is ugly but fascinating, and often leaves writers who spend a lot of time on screenplays out in the cold. This is not a small deal, either. A screenwriting credit comes not only with your name on the movie poster and on IMDb, but potentially huge sums of money in residuals. If the Writer’s Guild of America doesn’t put your name on a screenplay, you get no money.

The thing is: The way that screenwriters are assigned credit on screenplays often doesn’t make any sense, and when disputes arise, the WGA assigns three arbitrators to determine credit. The arbitration rules, however, often don’t make much sense, either, and, in many cases, writers who weren’t necessarily huge contributors to a screenplay get sole credit, while those who made the most contributions can often get no credit at all.

Here are 13 incidents where screenwriting disputes led to what many would argue are unfair results.

12 Years a Slave

The confusing, nonsensical rules are exactly why Steve McQueen, the director of the Oscar-winning movie 12 Years a Slave, wasn’t credited as a screenwriter. It’s often very difficult for a director to be assigned a co-writing credit on a screenplay because, in such instances, the director has to provide evidence of “considerable” contributions, according to the WGA. John Ridley, who adapted the 12 Years a Slave screenplay on spec for free with the promise of eventual compensation, was not willing to provide a co-writing screen credit to the director, McQueen, in this case, despite his many contributions. McQueen took the case to arbitration, and the WGA sided with Ridley, giving him sole screenwriting credit, even though McQueen essentially co-wrote the script. (This led to a well-publicized feud during the Oscar race in early 2014).

Up in the Air

In a situation very similar to the one involving 12 Years a Slave, Sheldon Turner brought the Up in the Air project to the studio, basing it on the Walter Kirn novel, but Jason Reitman disregarded Turner’s script and came up with his own screenplay, saying that he’d never even seen Turner’s script. Reitman’s script also incorporated some elements of Kirn’s novel, and, ultimately, Turner and Reitman’s script had substantial similarities. They were co-credited as writers, although Reitman wanted sole screenwriting credit. It wasn’t given to him because of the similarities (which are unavoidable when adapting another person’s work). When Oscar time came, both men accepted the Oscar. Reitman, though not happy with the outcome, nevertheless allowed Turner to speak first. For many, it was the first time anyone realized that Reitman wasn’t the sole writer on Up in the Air.

X-Men: First Class

In most cases, no matter how little of the screenplay is eventually used, the very first screenwriter to produce a script will receive a screenwriting credit. This was the case in X-Men: First Class, another dispute involving Sheldon Turner, who drafted the original X-Men Origins: Magneto screenplay. Though several more screenwriters were brought aboard, and the project was essentially started again from scratch when Matthew Vaughn came aboard, Turner was still given story credit on the screenplay. However, two other screenwriters who were involved early on — but not first — were not: Josh Schwartz (The O.C.) and Jamie Moss.

Jurassic World

Though Rick Jaffa and Amanda Silver wrote the first draft of the Jurassic World script, the eventual director, Colin Trevorrow, trashed the original script and rewrote his own from scratch and didn’t base it on their script. In fact, Steven Spielberg delayed the movie so that Trevorrow could pen the script. Nevertheless, though Jaffa and Silver had not done any work on the Trevorrow script, they were co-credited as screenwriters, a decision with which Trevorrow disagreed, although he did not ultimately appeal.

The Cable Guy

Though Lou Holtz, Jr. wrote the original draft of The Cable Guy, Judd Apatow came in later and substantially altered the film, giving it a darker edge, and turning it from a What About Bob? kind of film into a Hand that Rocks the Cradle kind of stalker movie. Though he brought it to arbitration to argue that he’d made considerable changes to the script, the WGA — much to Apatow’s chagrin — refused to give him any credit at all. Apatow was a producer on the film, and the WGA has a rule that, if you’re a producer, you have to meet a very high bar in order to receive credit. Apatow, under the WGA rules, didn’t reach that bar.

The Rock

Despite the fact that Michael Bay worked closely with Jonathan Hensleigh for nine months on the script for The Rock, the WGA decided in its arbitration hearing to deny Hensleigh screenwriting credit in favor of David Weisberg and Douglas F. Cook, who wrote the spec script. Bay was so upset with the WGA’s ruling, he wrote an open letter calling the WGA arbitration process “a sham, a travesty.” He claimed that Weisberg and Cook had a really cool idea, but that had he directed the movie they wrote, it would’ve been terrible (some may suggest that the movie he did direct was terrible).

Fear and Loathing in Las Vegas

Alex Cox and Tod Davies may have written the original adaptation of Hunter S. Thompson’s book, but director Terry Gilliam and Tony Grisoni rewrote the entire script from scratch. Despite that, Gilliam and Grisoni were initially denied credit (because as director, Gilliam was considered a producer), although the WGA eventually relented, giving them credit. Nevertheless, Gilliam was so frustrated with the process that he resigned from the WGA and burned his WGA card.

Ronin

Though the entire screenplay was written by David Mamet based on a story by J. D. Zeik, after arbitration, Mamet was only given credit under a pseudonym. A similar dispute arose over his contributions to the Wag the Dog script. After the incident, Mamet vowed to only work on screenplays in which he was the sole writer.

The Hangover

Despite disappearing for four months to rewrite the screenplay, director Todd Phillips and Jeremy Galecki were denied screenwriting credits, even though they incorporated into the screenplay the baby, Mike Tyson, the cop car, and Zach Galifianakis’ entire character. Nevertheless, Jon Lucas and Scott Moore received sole credit because — as producers — the bar was too high for them to claim “considerable” contributions. A similar thing happened on Phillips’ Due Date: Robert Downey, Jr., made contributions to the script, but was denied credit because, as a producer and the lead, the bar was too high.

The Expendables

After David Callaham received a co-writing credit for The Expendables by the WGA, Sylvester Stallone and the producers of the film sued Callaham for fraud, claiming that he overstated his role in writing the film. Stallone claimed that he only based a few of his characters on a 2002 script that Callaham had written. Nevertheless, Callaham received a “story by” credit and top billing as screenwriter, much to Stallone’s dismay.

Indiana Jones and the Kingdom of the Crystal Skull

Though it never actually went to arbitration, Frank Darabont threatened to take it there after George Lucas scuttled his entire script despite the backing of director Steven Spielberg and despite working on it for a full year. Nevertheless, Darabont claimed that parts of his script made it into the film. However, there’s no record of an actual arbitration suit. David Koepp, Jeff Nathanson, and George Lucas were credited as the screenwriters.

Spider-Man

Though several writers worked on the Spider-Man script throughout its years of development in the ’80s and ’90s — including James Cameron and a host of others who came up with most of the ideas in the film — the WGA ultimately decided to credit only one man for the screenplay, David Koepp (one of the same writers credited with the screenplay for Indy IV), even though his actual contributions to the script were very minimal.

Leatherheads

George Clooney nearly quit the WGA in his dispute over screenwriting credit on Leatherheads after he was denied screenwriting credit on the film, despite the fact that, according to Clooney, only two scenes from the original screenplay written by Duncan Brantley and Rick Reilly made it into Clooney’s film.

(Via TheWrap, the L.A. Times, THR, Deadline, Vulture, Smart, Popmatters, WeMinoredinFilm, Variety)