I agree with the couple of other responses. There’s a color difference in the background of the response heading, grey for the meaning in English, and black for the reading in hiragana. Also, if you notice that your English shows up in a weird combination of hiragana and single letters, you’re probably doing the other thing. That said, I still make mistakes like this a couple of times a week, and I’m level 27, so it’s not like you’re a crazy person or anything.
Or do what basically all of the rest of us do and install a cheat Tampermonkey script so you can undo a wrong response, and you can just take it back when it happens.
Wanikani Override: /t/Wanikani-Override-Userscript-112-Ignore-Answer-Button-No-longer-supported/2846/1
The big list of addons: /t/The-New-And-Improved-List-Of-API-and-Third-Party-Apps/7694/1