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.