In most cases, draft mistakes more negatively alter the course of an NFL franchise than any other mistake.
Mishaps in free agency happen all the time, and teams don't have a problem getting out of them. Look at the Jacksonville Jaguars already dumping Nick Foles on somebody else.
The same doesn't apply to draft mistakes, where a precious, premium asset goes to waste on a bust. When this happens, teams dramatically fail to fill problem areas and also have to deal with the hindsight perspective, which showcases which prospects they should've drafted instead.