Can't be overestimated/Can't be underestimated - which one to use?
There has always been some muddling up of the words overestimate and underestimate. The use of the negative in the expressions 'can't be over/underestimated' makes it easy to slip up. Also, the fact that, when you use either phrase, the context of what you are saying usually makes your meaning clear. Many speakers (and the people they are speaking to) just don't bother to check.
Here's an example I heard on the radio today (on the BBC news as it happens):
"The influence of Google in the Internet world can't be underestimated".
Really? I'm sure it is pretty easy to underestimate it. What the reporter meant to say was 'can't be overestimated'.
An easy way to get this right is to remember that reason things can't be overestimated is because they are big/important and the reason they can't be underestimated is because they are small/insignificant. In fact, that's all there is to it - you can substitute 'is big' for 'can't be overestimated' without losing the meaning and likewise with 'can't be underestimated' and 'is small'. Had the BBC reported that "The influence of Google on the Internet world is small", they might have got letters. As it is, 'can't be over/underestimated' go their own way and, apart from with those who like to get these things right, sneak under the radar.
Of course, purists might say that even something large can be overestimated. Google's influence is big, but it could be bigger. However, that may be taking things just a step too far.