What happens when a word that you know is the correct answer doesn't fit in the crossword?

First, you get frustrated. Next, you check the origin of the puzzle...is it using British English or American English? That could change the spelling. After that, you check to see if the Words that cross might be wrong. You could then look for synonyms, if possible (which it isn't always, especially if your answer is a proper noun.) After that, I usually throw the paper across the room or write a nasty letter to the editor (which I don't send, but I assure you contain many fine vocabulary Words.)