I think duo could be a little more benevolent sometimes.
There are a few mistakes that are easy to commit when you're not a native english speaker but you are using english to learn another language. Well, your focus isn't on your english speak, it is on the new language you're learning, so it's easier to commit mistakes that perhaps one, as a native, wouldn't commit just because it would sound SO wrong or unnatural, that it just wouldn't happen. Foreign speakers don't have that tone in the ear, so in these situations we're likely to commit more mistakes.
Duo's can already feedback the nature of the mistake most of the time. I think that instead of failing you at once, the minor mistake could be fedback to the user, so he can fish for mistakes, before telling him out.
Here are some examples that illustrate my idea.
If one confuses gender, the system can say "are you sure about the sex of the child?"
If one forgets a word: "You a word"
If one mistakes prepositions: "Wrong preposition"
If one mistakes the arcticle (a/an): "Choose an other arcticle"
From the top of my head, these are the examples I can think of. These are silly mistakes, that are just too frustrating.
(these, of course, only apply when translating from french to english)
I sometimes hit the enter key by mistake half way through answering the question. Really annoying but Duo just marks it as incorrect and I say to myself, don't be stupid in future.
Duo then gives me a chance to get it right in the following questions. On the old hearts sustem, if you got it wrong on the last question and you were on your last heart then it was frustrating but having to repeat the exercise was good because the more practice you get the better.
How would Duo know your error was a silly mistake and you really knew the answer?
I find the zero tolerance of spelling errors and gender confusion oppressive to the point where I've given up on the tree for the language I already know. It's too rigid to allow you the chance to improve and wastes so much time. I wanted to improve at those things but it's a disproportionate focus on one aspect of the language at the expense of more important things - I'm not at all sure how natural the constructions are that the site allows (or indeed teaches) and how far anyone is being taught to understand their natural application.
The buzzword in language teaching has for a very long time been 'communicative'. A course is only good if it's communicative. Yet communication skills seem to be the very thing Duolingo loses sight of at times.
"How would Duo know your error was a silly mistake and you really knew the answer?"
well, it doesn't. But if you're studying another language, its purpose is not to correct you on your, English, though. "A House", "an house", what's the deal?
Well, one could always say that duo could just let you know and let you review the phrases yourself in these cases. But I think that straight up shutting someone down may be counter-productive.
Well, I'm a native English speaker, and I am taking French for Spanish speakers. I admit, it is really frustrating to miss a sentence because of a dumb mistake in my Spanish, because I am focused on the French. I might truly understand the French "la robe", but get marked wrong because I put "la vestido" instead of "el vestido", which I would NEVER do when speaking or writing Spanish, but I tend to do when translating from French to Spanish because the French is feminine and I put a feminine article and then translate "robe" to "vestido" correctly.
But at the end of the day, I chose to learn French for Spanish speakers precisely because I want to improve my Spanish while I learn French. And although my progress is slower, I do feel that I am learning a lot.
So instead of thinking of it as wasted time because of silly English mistakes, try to think of it as time spent improving your English to a more native level. Besides, a little more practice never hurt anyone.