I'm curious, what kind of error are we talking about? Words that don't exist, or another solution to a problem that may not have a unique solution?
- There was one with a name that suggested an Alice in Wonderland connection, and it had an answer "THE LOOKING GLASS" (no spaces) running vertically down the full length of the center of the grid.
Every across answer that was entirely to the left of that was written normally. Every across answer entirely to the right was written backwards. Every across answer that crossed the center was a palindrome centered on the center.
- There was one where several answers were triple Spoonerisms of well known phrases.
For example, the answer "THE STUCK HOPS BEER" for the clue "Tagline in an ad for Elmer's Glue-Ale". Rotate the ST from STUCK, the H from HOPS, and the B from BEER and you get "THE BUCK STOPS HERE".
- I remember one that had a few isolated black squares, and a theme that suggested the puzzle had something to do with roundabouts.
Those black squares were roundabouts. Answers would hit the roundabout and continue after a 90 degree turn.
- I remember one where the theme was something like "What goes up must come down". That had several answers that, like the roundabout one above, would take a 90 degree turn from across but it was a left turn so after the turn they went up. Then there would be a down answer whose start was the reversed end of that across answer that would come down, also make a left tern, and continue across to the right.
I'm interested in your magical solution because this is a hard problem.
So it can't simply blindly reject answers that are non-existent words.
An example of how this computer could make a mistake: Sometimes two proper names are crossing at a vowel. If you don't know either name, you sometimes have to guess blindly at the answer. (This exact scenario is rare in crosswords like the Times, but do occasionally come up, and similar scenarios exist.)
Either way, this is all moot when discussing a crossword puzzle contest, which explicitly limits itself to words in a specific dictionary + proper nouns. The problem of proper nouns is still extreme, and brings down the whole idea, but at least the problem of recognizing 'all possible common nouns that could be present in the crossword contest' is simple.
My favorite example of using this is the 1996-11-05, the day of the presidential election, NY Times crossword with the clue "Lead story in tomorrow's newspaper (!), with 43-Across". The puzzle worked with both "CLINTON" or "BOBDOLE" in the crossword.
However, with a combination of a bad clue and a bit of bad luck it does sometimes happen that an experienced crossword solver has to take a guess before sending in their solution. I'm not an experienced crossword solver, but I've seen them at work, and I would guess that finding a good alternative solution happens less than one time in 50. That's just a guess, though. For a better estimate one should systematically compare the solutions produced by competent crossword solvers with the official solutions.
Of course, people don't like to play crosswords like sudoku, blindly matching letters, so there does have to be some skill to the clue design to have a relatively small amount of plausible answers, if not being entirely unique.