What Herman Cain did in Orlando this weekend was both completely unexpected and entirely predictable.To this Scott Aikin replies that it can't be true. But of course it can: the situation in which something was predictable but unexpected is the situation in which it could have been predicted (and thus was predictable) but no one actually did predict it (and therefore it was unexpected). Since such things do happen, the original statement is entirely fine, logically speaking; any tension is the deliberate rhetorical tension that arises from foiling expectation. And it's clear from the rest of the paragraph in which this sentence is found that this was precisely the point: the idea is that nobody expected it, but given what it would take to win the straw poll, it was "in some sense inevitable".
I don't think this is a silly error, though, since you could very well read 'predictable' as saying that it was predicted (and thus expected). What it points to is that there is a whole class of ability-words and ability-phrases that are potentially ambiguous, being the sort that can be read either as indicating mere potential or as indicating activated potential. The best known and most widely used example is 'visible', which can mean either that something is see-able, or that something is seen (and thus see-able), but there are a number of others: sensible, intelligible, and so forth. Interesting, most of them are broadly cognitive in character, although you can occasionally find them in other contexts -- in the Aristotelian terminology of Aquinas's Third Way, for instance, the 'possible to be' and and 'possible not to be' is elsewhere shown to be precisely such a phrase, intended to be read in the activated-potential sense; it's a way of understanding the phrase we don't typically use anymore, which is why it takes some effort to read the Third Way in the sense it was intended to be read.