When I answer something incorrectly, the website marks it green as if it were a correct answer. Note the red X at the top indicating I answered incorrectly.
There seems to be a logical root to the problem: the coloring system knows to mark the correct word as green and anything else red, but when you answer a question incorrectly it displays the correct word in the cloze and thus the coloring system sets it to green.
While this isn’t a gamebreaking bug, it certainly is annoying and forces me to do a doubletake since I notice the word color quicker than the little red X at the top and thus often initially think I answered correctly.