I'm currently reading a very good book: Schrödinger's Killer App: Race to Build the World's First Quantum Computer by Jonathan P. Dowling. Eventually it will show up in the book feed of this blog. But I hit a speed bump on page 228:
The role of quantum computing in the field of quantum technology cannot be understated.
That's a specific example of pretty common idiom. I hate it. Because I always read it as a challenge. I wanted (in this case) to tell the author:
Oh yeah, smart guy? Watch this: "The role of quantum computing in the field of quantum technology is nada. Bupkis! Zilch! Less than nothing!"
Mission accomplished. I understated.
But it's worse. The idiom's general form is something like
The importance of X cannot be understated.
Problem: you sometimes see it as:
The importance of X cannot be overstated.
These should mean opposite things. They don't. (It's not like the folks who say "I could care less" when they mean "I couldn't care less.") Instead, they are both trying to say
X is, like, really important.
… but in a rhetorically inflated way. Writers, speakers: just don't do this.
But (I should add) it's not just me. This Atlantic "Word Court" column from 2004 weighs in for a confused reader:
Cannot understate and cannot overstate are like architectural elements in an M. C. Escher drawing: if you like, you can flip-flop them in your mind. The trick is done by cannot, which has two meanings. Think of Parson Weems's tale in which the young George Washington declared, "I can't tell a lie." Of course Washington was physically capable of uttering a false statement; by can't, he meant he chose not to. Can't, or cannot, can mean something very much like must not—and if it means that, cannot understate the importance of makes sense. Clear communication is subverted, though, when antithetical statements mean the same thing. Cannot overstate is more commonly seen and heard, as you say—in fact, it's much more common. Why not use it from now on?
Or (my advice) just use neither, and say clearly what you really mean instead.
But wait, there's more. Linguist Mark Liberman followed up on the Atlantic column at the very scholarly Language Log here. Ready to leap into the weeds?
In an earlier post, I related examples like cannot understate the importance of... to the hypothesis that it's hard for people to calculate the meaning of phrases involving negatives in combination with modals, scalar thresholds and so on. This interpretive difficulty explains why some phrases with semantically-backwards interpretations are hard to edit out -- it's hard to calcuate what they actually mean, and they include pretty much the right words, and they're syntactically correct. In order to explain why the erring phrases are constructed in the first place, I suggested combining this interpretive difficulty with a sort of lego-block model of sentence construction -- take out an assortment of relevant tree-fragments from the lexicon, and fit them together until it looks OK. Sometimes another factor may be a sort of semantic gap, created by the fact that there is hardly ever any reason to want to express the idea that corresponds to the correct interpretation of the phrase in question.
Liberman goes on to use terms like the "modality of moral obligation" and "deontic necessity". Fine. You, like I, might have better luck understanding Dowling's explanation of quantum entanglement.