While I don't know the innards of it I suspect the typos are preprogrammed. Note what frequently gets marked wrong and have a human decide if it's a wrong word

While I don't know the innards of it I suspect the typos are preprogrammed. Note what frequently gets marked wrong and have a human decide if it's a wrong word or a typo. You could also do a string distance comparison--if it's distance from the right answer is 1 but it's at least two from any wrong answer it's probably a typo.