I've grumbled about that one before on here - that error turns the literal meaning completely on its head: "You can't underestimate the crucial importance ... " would mean that there is no bottom to the estimation of the unimportance of the "crucial importance", which is nonsensical as well - whereas "you mustn't ... " emphasizes how "crucial" the "importance" is.

One I've noticed lately that amuses me is the frequent "it's going to be much bigger than we expect", or variations thereof - which could be re-worded as, "we expect it to be much bigger than we expect it to be" ... !