Breffni O'Rourke writes:
Here's one for the 'cannot underestimate' files. The publicity material for the recently published Atlas of the Great Irish Famine (which may coincide with the printed blurb or the preface; I haven't been able to check) starts off with (variants of) this:
The Great Irish Famine is the most pivotal event in modern Irish history, with implications that cannot be underestimated.
From the book's website:
The Great Irish Famine is possibly the most pivotal event/experience in modern Irish history. Its global reach and implications cannot be underestimated.
From a feature article in the Irish Times by one of the book's editors:
The Great Irish Famine is possibly the most pivotal event in modern Irish history and its global reach and implications cannot be underestimated.
The 'cannot' = 'must not' interpretation ('Don't underestimate the implications of the Famine') certainly doesn't work in this context. It would be strangely defensive to introduce a landmark work of scholarship by mentioning doubts about its significance, even if only in dismissing them.
Update — An interesting trend is visible in the Google Books ngram corpus:
Based on a spot check, nearly all of the "X cannot be underestimated" examples are either ways to say that X is important and should be recognized as such, or are part of a discussion of whether this way of phrasing that thought is correct or not.
Some related (?) trends: