The biggest mistake fantasy football owners can make before draft day, especially in formats with long-term impacts like dynasty and keeper leagues, is failing to prepare themselves for the wide range of potential outcomes through the use of mock drafts.
Everybody has favorite targets and it becomes easy to focus in on that set of players during each mock with hope the actual draft plays out the same way. But when things start to get hectic during the draft, and they usually do, it's imperative to have several alternative plans of attack.
It's also important to test each hypothetical from every area of the draft order (early, middle and late) to ensure maximum preparation.