I'd enhance that a little and say that it's easy to think of facile reasons it won't work. But let's say you think of a reason why a thing won't work after two minutes. If someone else has been working at the problem for two hours, or two days, or two months, or two years, what's more probable?
- They never made the initial connection to the flaw, and have failed subsequently to notice it or account for it.
- They thought of a solution to the flaw which you haven't.
Hrm. Maybe I just don't have that high of an opinion of my own faculties, but I tend to assume it's the latter. Especially if the person in question is a very intelligent person with a strong track record of outperforming expectations.
That doesn't mean you shouldn't probe, or seek clarification, or ruminate on the flaw. But if you tell someone who's been working on a tunnel boring problem for a whole quarter that "boring through rock is the most expensive way to connect 2 places (sic)" as at least one person in this thread has observed, I'd argue that that's not a useful reaction. My reaction is, "I wonder what he knows that I don't."