I use Tsurukame to do WaniKani reviews and it shakes its metaphorical head (the text input shakes back and forth) instead of submitting the answer when I make a mistake like this. It’s saved me on a few occasions.
お客さま、お客さん、客さん、観光客… My head is spinning just thinking about it, so I’m sure that I regularly misuse these and other variants. But, fortunately(?), I am super slow and methodical when it comes to study (I’m on the 10 year plan…) so I at least get things (I know) right when studying.
Hi there! Thanks for sharing this with us. I will pass the feature suggestion along to the team but for now, feel free to send specific suggestions like this our way so we can take a closer look.