needle in a haystack...

...or just one puzzle piece among 3000?  My husband and I decided to start putting our 3000 piece puzzle together last night, and as always we begin with the edge pieces to form the border. But after about an hour and a half of working on it we are still missing 2. Even though we weeded very carefully and systematically through all the pieces we must have somehow missed those two.
And because looking for them among the couple thousand that are left feels a little like looking for a needle in a haystack, we will simply have to wait till we find them as we start putting the rest of the puzzle together.

I feel like I should be drawing some clever analogy here, but none are coming to mind.  I'm sure as we keep working on the puzzle, analogies will abound. Puzzles have a habit of doing that...