I remember being surprised by the kanji lesson at level 51.
With the skill level I had at that time, at that point every single kanji were completely new to me… but at level 51, suddenly there was 5 kanji I knew already:
沸 (Boil), 濯(wash) , 燥(dry up)
Those 3 kanjis are written on device like bath or toilet in Japan, so I see them literally everyday.
瓶 (bottle)
玄 (mysterious) The kanji is rare on its one, but the word 玄関(げんかん) is super common
and while it might not seem like the easiest kanji to learn….prison (lv 35)
出獄【しゅつごく】release from prison
入獄【にゅうごく】imprisonment
地獄【じごく】hell
脱獄【だつごく】jailbreak, escape from prison
…because they all make perfect sense and just builds upon the kanji you already know, with no strange readings or rendaku (a huge relief at that point).