Guess what. WaPo pointed out that exactly the same thing happened in South Carolina.

It's worth noting that, while this was a big miss, it's not that unusual. Primary polling is often somewhat hit-or-miss, as most people understand. But this is also a function of the amount of attention being paid to this particular race. Had Clinton been expected to win and gone on to win by a much larger margin, had it not been primetime on a weekday, we might not have noticed.

You don't think so? Well, the polling in South Carolina was exactly that. It missed by 20 points versus the Real Clear Politics average on that Saturday, underestimating Clinton's massive win in the Palmetto State.

(...)

The other reason we're so focused on Michigan, of course, is that Clinton was supposed to win big but lost. Clinton should have had a big win and taken a big step toward the nomination. She didn't, and the polls didn't catch it. We very naturally don't care that Clinton's South Carolina blow-out was a mega-blowout. We do care that we didn't know who'd win a state.