Oh, I could do this for HOURS - I've thought about doing a video series called
Bombs Away covering how certain movies bombed
As for some examples:
John Carter (2012): The original title of John Carter of Mars is kept, as is the original release date in June, and PR is much handled better.
The 13th Warrior (1999): John McTiernan and Michael Chrichton get along, thus saving the film production difficulties and stays on budget. Much the same, it is not released against The Sixth Sense.
The Last Stand (2013) - it is released any month outside of January
Scott Pilgrim vs. The World (2010) - Michael Cera is never taken as the lead actor, instead, the role is given to Aaron Taylor Johnson, who hot off the heels of Kickass, gives the film much needed positive PR as opposed to Cera's negative PR. The studios move the release date from the highly competitive August release (it got butchered by The Expendables and Inception) to a far easier October/November release.
Rise of the Guardians (2012) - released on Christmas, so Wreck-it Ralph doesn't compete for tickets.
Heaven's Gate (1980) - United Artists puts a tight leash on Cimino, and the film, while not a smash, is not the horror story is is OTL.
Sahara (2004) - Budget is kept on track, better use of marketing, released latter in Summer than it was.
The Alamo (2004) - released on time, allowing it to catch some of the post 9-11 jingoistic tail winds.
Green Lantern (2011) - The movie is not centered on Hal Jordon, but John Stewart, with Anthony Mackee cast, and is formally announced as the launching point of a formulate DC Universe. The more action-oriented focus brought by Stewart (being a Marine) and the dual PR storms over the first major black superhero and DC finally getting in the game with Marvel helps it double its budget and more.
Waterworld (1995) - Kevin Costner is not given absolute control, but instead, direction is given to one of the screenwriters - Joss Whedon. The budget is kept much smaller, the script much tighter, and the movie is a surprise hit.
Mission to Mars (2000) - the Script is given some polish, and gets better reviews as a result.
I can name more, often times, its the smallest of things that tanks a film. But there are a number of bombs - Cutthroat Island, Mars Needs Moms, After Earth, RIPD - simply cannot be saved.