The films above fit the "multiple weeks" description, which is a worry for projects that started off with huge budgets in the $100-$200 million range. One reason for the reshoot trend is studios' unwillingness to contend with finicky and expensive directors with experience (think David Fincher or Michael Mann); they prefer to bring on relatively inexperienced (cheaper) directors who they can control. For "Ronin," Universal selected first-time director Carl Rinsch, a choice that is now leading to an extensive reshooting of the film's final battle scene, which Rinsch closely supervised. Universal knows that a franchise can be saved via reshoots--as long as it's a savvy producer like Frank Marshall, who helped to turn Doug Liman's "The Bourne Identity" into a lucrative winner.
Of course seasoned veterans can have problems, too. For Marc Forster's "World War Z," writers Damon Lindelof and Drew Goddard were pulled in to re-work the film's third act. Forster helmed 2008 Bond installment "Quantum of Solace," so he has the wherewithall to pull off a big production, but a major seven-week reshoot is in the works for "World War." And a major star's career is at stake: Brad Pitt may be willing to take a flyer on a small indie film he does for cred--but not a big-studio movie intended to burnish his marquee value. The other question is whether Pitt is even speaking to Forster.