Creator Backlash/Old Shame: The overwhelming majority of the cast has since gone on to display regret for being involved with the film. That many of them were roped into it against their will didn't help.
Deleted Role: Several (see What Might Have Been). Also, Bob Odenkirk directed two segments that didn't make it into the US cut of the final movie, but is nonetheless still listed in the credits.
Old Shame: With the exception of Farrelly, no one involved seems particularly proud of the finished product.
At the film's premiere, the only actors who showed up were Seann William Scott, Chloe Moretz, Common, and Jimmy Bennett. Others, like Elizabeth Banks and Kristen Bell were going to attend, but dropped out at the last minute, with one magazine claiming they were "too scared." And if Scott's red carpet interview was any indication, it sounds like the only reason he went was because he was hoping to see Halle Berry.
Playing Against Type: This may be the only time you'll get to see Naomi Watts dress up like a high school teenager and calling some kid "fuckface".
The Shelf of Movie Languishment: The film began production in 2009 and was intended to ready for a 2010 release. Then original distributor Overture Films sold the rights (and later had everything at the studio but the name sold) to Relativity Media, who continued to keep it in-and-out of production (missing a planned Spring 2012 date) until finally releasing it in 2013.
The framing device was initially supposed to involve three teenagers going on a treasure hunt to find the mysterious Movie 43 of the plot and the rest of the segments would be featured as what the characters thought was it but wasn't. It seems that got dropped in favor of adding more cameos.
The version of the film released in the UK apparently retains this device instead of the 'movie producer' version.
Sam Rockwell was originally attached to reprise Batman in the superhero segment (which was intended to be a sequel to the short film Robin's Big Date) but pulled out to do Seven Psychopaths instead.