Is the glass half empty or half full? :) It's a work in progress with easily extensible rules http://proofreadbot.com/rules
Will put something up for wont. :)
Don't get me wrong, it looks like a great tool and I shall try and use it myself for my own studies, I just found that particular error amusing and shows you can't just trust even your own bot.
Also, errors are something that Proofread Bot wont put up with.