"SO" is not a valid word, but I don't see anything in the instructions saying words must be at least three letters.

Edit: Perhaps you could add a toggle for the list of words, to switch between the order they were found in, and alphabetical order? This would be particularly useful when there's an S in play, to find words that could be made plural.

I enjoyed it and I have one suggestion: a shuffle button.

really gets you thinking on a few of them, it's fun