English is a fantastically flexible language that can be bent to express all manner of things. However, care needs to be taken because it is all too easy to bend it back and say the opposite of what you mean.
I noticed this in a story on the BBC website yesterday, entitled Online advertisers change tactics. Towards the end, it says:
“When it comes to online advertising you cannot underestimate the power of social networks,” said Fadi Shuman.
I don’t know whether the speaker or the columnist is to blame but I am sure that the sentence doesn’t convey the meaning they intended. Perhaps “should not underestimate” or, if they were feeling hyperbolic, even “cannot overestimate” but not “cannot underestimate”. The thrust of the passage is that online social networks are an incredibly powerful medium for advertising and, here, the argument seems to temporarily gallop in the opposite direction.
Of course, it is all very well to joust at other people’s mistakes; the challenge is to avoid being hoist by my own petard!