There could also be an extra "unseen" character like a space.
Or, it could be that the enter button was being pressed after the answer was entered.
Are you sure that you didn't just accidentally type 8 or 0? I've never seen it get its own sums wrong like that before. Did it just happen once or is it being consistent?
It only happened once.
Dreams don't come true, you gotta make them come true.