(Un) acceptable failure rate

There is as userscript that upon wrong review will show you possible candidates you confused with:

It helps to constantly remind oneself that failing a review is not a bad thing. It is good since you will learn the particular item better. As you progress, there will be more and more examples of the same English meaning represented by 2-3 different kanji, even more if you include vocab into equation, not to mention lots and lots of “visually similar kanji”. Each confusing pair or triple may require extra effort to master. Thankfully forum is full of helper threads where people share their tricks.