The biggest mistake was easily Perry's "Oops" debate gaffe. And probably a handful of other Perry gaffes I cannot think of right away (let's re-invade Iraq? That odd speech he gave in New Hampshire where he seemed like a gay southerner on pain killers?). Without those, Perry may have ended up winning Iowa and carrying that momentum into South Carolina, leading to a well-financed showdown in Florida and beyond.
The biggest post-Perry mistake goes to Gingrich for doubling-down on the Moon Base.
Santorum's biggest mistake was probably doubling-down on anti-contraception.
Romney's biggest mistake was running away from his record. It would have been an ever worse mistake for him to run on his record.
