@FoxxFireArt: Hmm.. this riddle is indeed challenging. I LIKE it!
Alright, I am guessing that the error is the phrase "It's impossible" since 80% of the students COULD NOT find the error, however 20% of the remaining students DID find the error. Most likely there really isn't any error with the alphabets. Its just how the third line in regards to those students not finding the error and the phrase "it's impossible" to find the error is worded. Hope this is it.