Truth be told, the committed delegates in BOTH parties are actually free to vote their will. Since they are selected for their strong support of the candidate that won their state, they rarely do other than the voters directed them to do, but they can.

One scenario where that might happen, and where BTW it ought to happen, is if a horrendous corruption or vote-rigging scam was discovered after the primaries but before the convention. In such a case, the delegates (super or not) would all be crazy to just vote for the soiled candidate anyway. They could reasonably assume that, had the voters known then what is known now, they would have cast their votes differently.