Can you challenge a variation of a word that you used earlier in the game?

I used a word and my opponent didn't challenge it. Then my opponent added an -S to my word and then I challenged it and it turned out that it wasn't a word in the Scrabble Dictionary. Am I allowed to challenge a variation of a word that I previously used?

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A good question. The answer is most definitely 'Yes'. You can challenge any new word formed by your opponent, even if the newly formed word is a 'variation' of one that escaped detection earlier. The fact that you got away with playing the singular form of the word earlier in the game is completely irrelevant.

So suppose you played the incorrect word CLOM*, and your opponent doesn't challenge it. Later in the game your opponent puts an -S on this word to form CLOMS*. You are welcome to challenge this word, and your opponent has to take back their letters and miss their turn.

In fact, I've seen some bold tournament Scrabble players apply this ploy on purpose. They'll play a false, but plausible, word against an opponent who doesn't challenge very often, and then challenge it when their opponent pluralizes it later in the game!

It can get nasty in the upper echelons of this family game ;-)

Hope this answers your question,