We all like to remember our successes and forget our failures, and finance is no different. As investors’ inboxes once again become clogged with annual outlooks from Wall Street’s scribblers, there is little admission of the nearly universal failure to predict what happened this year—even though the things the analysts missed are much more interesting than their forecasts.

There are two big lessons to learn from the mistakes of the year-end crystal-ball gazing. The first is that when everyone agrees that prices can only go...