I was trying to type in りょこう to answer for 旅行, but when I pressed enter it just shook at me with no note.
I tried a few other forms of it such as adding an う but it still wouldn’t let me press enter, only endless shaking.
I ended up being able to bypass it by typing my go to when I can’t recall some thing which is れ.
Very frustrating to not be able to get the correct answer and having to purposely get it wrong to proceed with other reviews.
First question is always, do you have multiple tabs with WaniKani open, or do you have scripts running?
Second, can you recreate the behavior?
Once you are past those, giving the details of the device and environment you’re using would help.
Just the one tab, and I don’t use any scripts. I’m using firefox on a linux mint system, and once the same word came up in the review cycle again I was able to pass it normally.
Perhaps it was a weird one off, but it was still quite frustrating.
If it happens again, I would recommend refreshing the page so you don’t have to put a wrong answer in, but without more information I’m not sure the staff will be able to find a cause for that specific instance.
Thank you for replying, at the least.
I know it’s not the most thorough bug-style report, but perhaps if someone else encounters something similar, we may find more data.
If this happens again for whatever reason, consider installing [Userscript]: Double-Check (Version 2.x). As long as you know you won’t abuse it, it can help with simple things like this.
Having a second window with reviews open or accidentally hitting the space bar before or after you input your answer for the reading question will cause wanikani to shake your answer.
A screenshot would help as well, since maybe others would notice what was wrong. I wonder if you had a leading space or something like that.
I assume you accidentally had a space in there. If this happens, delete and make sure you see the light gray 答え in the answer box indicating that there is nothing in there.
The same thing seems to happen to me sometimes. I use multiple scripts so it might be because of those.
I doubt this is a script or WK issue. As @seanblue and @sadfghjkl have already said, most likely there was simply an empty space at the beginning, and then WK won’t accept the answer.
It is possible - I’m pretty sure I deleted everything typed and then retyped it from a clean box, but I was fretting, so can’t remember clearly.
Hi @Ararothea! WaniKani will shake without a message if there’s a space, as others have already mentioned, and also if there is romaji or kanji in the answer field. It doesn’t sound like romaji or kanji, though, so a space is the most likely explanation. Please let us know if this happens again and deleting and retyping doesn’t fix it. As seanblue mentioned, a screenshot or (even better!) a video would really help us to get to the bottom of it! Sorry for the frustration 
This has happened to me a couple of times in the past as well, albeit with a different vocab.
I am also not using any scripts, nor had any other Wanikani tabs open at the time.