Sounds stupid, but my most recent ‘mind blow’ was this:
絵文字
I think I once vaguely knew in a pop-culture-quiz context that emoji meant ‘picture character’ in Japanese. But then it became part of my English vocabulary, and I never thought about it again.
Imagine my surprise when I discovered my ‘English’ word contained kanji all along… pronounced エ,モ and ジ no less!
Also, learning the structure …し、…し to list non-sequential statements. For ages, I thought people were finishing all their sentences with し for no reason, and assumed it was some kind of slang particle.
Similarly with かな to express wonderment or のに to express disappointment. If, like me, you hear more Japanese than you study, you go through several phases of understanding with this kind of thing.
First 10 times you hear it:
I thought I understood what you said but then you made this random noise at the end…?
50-100 times:
OK, there seems to be a tone of voice / facial expression you’re doing too. What’s up with that?
Over 100 times:
I sense a pattern of mild (disappointment/surprise/anger/contentment/excitement). I’m going to copy that and watch your reaction.
After a few attempts:
The results are inconclusive. Am I talking like a girl? More data is required.
Finally you look up a grammar lesson:
Gah! Why did nobody ever explain this?
When you realize the vocab you’ve been studying is used in traffic signs, i.e. 
I don’t know if anyone else struggles with the stroke order for the ‘turkey’ radical
I recently stumbled upon this animation of the Han character on Wikitionary, now everything makes sense when you think of it as a compound radical. I never look at it the same way as I did before.
When I found out that Tamago means egg… and I had a Tamagochi when I was young… Which… was the shape of an egg…
it’s actually one of my fav radicals to write! it just flows so smoothly from the hand onto the page and always seems to come out rightly proportioned.
I just recently learned that instead of unmarried, in Japanese, you are not-yet-married (未婚).
Not exactly mind-blowing but an interesting cultural touch maybe? I guess, culturally speaking, getting married is just a matter of time?
I will never forget たまご now.
I think “tamagochi” is a portmanteau word of tamago and tomodachi (friend) !
It’s たまご and ウオッチ (watch).
rambo (movie) 乱暴 = violence. and a few other ones i’ve forgot ;s
My ultimate mindblow was when I first realized that ji in emoji is the ji in kanji.
I wonder if in a few thousand years there will be a wanikani for emoji 
Yeah, I always thought “emoji” had something to do with “emotions” or “emotes”. Turns out it just is “a letter that is also a drawing”, which is funny coming from a language based on ideograms. 
Almost no one says that word without お, but if they did, I’m fairly certain the pitch accent is different.
Hey, don’t spoil my fun! 
This is actually a great mnemonic, imho.
Well, you can go with 私利 if you want, that one has the same pitch accent.
New mind blow – I realized that 金玉 sounded A LOT like the title of a popular anime series, Gintama. My first reaction was "they didn’t . . . "
They did. It was intentional. I haven’t watched Gintama, but now I may have to make time for it.
I liked the life action movie too!
so… just learnt 指圧（しあつ）… Shiatsu = finger pressure…