[bloak]

In a traditional cryptic crossword each individual word has at least two independent things in the clue referring to it, for example, in an unrealistically simple case, "it means X" and "it's an anagram of Y", so each individual answer is very likely to be unique. In addition, alternate letters of each answer are part of another word, so the whole thing is extremely likely to have a unique solution.

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.