Sometimes you randomly get hard questions. I got caught out by one earlier.
Today I received the following Practice vocabulary question: 'Chemisty'. Two of the answer choices were 科学【かがく】and 化学【かがく】. Even though I had 'show kana' turned off, I read 科学 and instantly thought 'ah, that is read as "kagaku"; that sounds right...' and so I clicked it without checking what the other options were... Of course, this 'kagaku' (科学) = Science, not Chemistry! :/
If instead I had 'show kanji' turned off (instead of turning off 'show kana'), then that question would have been extremely difficult! :P