Ok, this has happened twice (or more), so there's probably a pattern.
Somedays I'll be working on something and totally go braindead on it. I get sleeepy, can't think straight, can't seem to settle on algorithms to use, etc.
It may be that on a sub-conscious level my brain knows there must be a better way, or this isn't right and just shuts down (either thinking, backgroundishly, about how the right way would be, or just shuts down as a preventive measure to keep me out of trouble.)
What are other people's experiences? How do my gentle readers know that the track they are on might not be the right track after all?