In japanese input reviews, if you type something like じつ, then hit left arrow twice to go to the starting position
_じつ then type any japanese letter, さ for example, it becomes
さじつ_ with the cursor at the END of the string, without you doing it. So if you wanted to correct じつ to さいじつ by appending from the start, you’d instead get “さじつい”. Your cursor got pushed forward after you typed the さ, and you just got a ding on 祭日!!!
This does not happen with Windows Japanese IME, just the wanikani input field.