Misunderstanding what the difficult part of a job is. Specifically, the misapprehension that the hard part of a given job is thinking of a starting point or basic approach to the problem at all, when in fact the hard part is getting all the implementation details to work right given such an approach. People under this misapprehension will cheerfully suggest a variety of starting points, some of which might even be the right one but certainly none will be a good one you hadn't already thought of; they will then be puzzled when you look more rather than less irritated, and mysteriously don't thank them profusely for their vital contribution and spring immediately into action.

In extreme cases of this, the person will provide several basic approaches and (implicitly or explicitly) suggest that you ‘simply’ try all of them and see which one works best, apparently blissfully unaware that they've just attempted to multiply your workload by three or four (or perhaps even succeeded in doing so, if they're your manager).