There was a bug; it's fixed now. A few of the exercises would say you were typing in English no matter what you entered, including copying and pasting the given "correct answer" in. Obvious bugs like this you can report with the little flag, and then choose "something else went wrong" and describe it in the text field. Or you can go to the bottom of the duolingo website and dig through the menus to find the actual bug report form.
Hence why I asked if it was a listening exercise. I've heard a number of users mention this before, but I've never come across it myself, nor have I seen screenshots of it happening, so I can't verify what the problem is.
I suspect it is a Duolingo bug, in which case the best thing to do is take a screenshot the next time it happens and submit it along with a bug report: https://support.duolingo.com/hc/en-us/requests/new