Fan Nickname: The "Omni-Directional Bitch Smack" from chapter 95.
God Never Said That: When chapter 83 revealed the cup size of the female students, it caused a Hype Backlash from the Rinka Hayami fans, because they knew it's impossible that Hayami has a D-cup. The Shounen Jump redaction admitted that their editors made a mistake and accidently switched Hayami and Sumire Hara's cup sizes (C-cup and D-cup, respectively). Unfortunately, the English fantranslators didn't correct that mistake.
Interface Spoiler: The table of contents in the eighth volume has one of the chapter titles scribbled over because it gives away the identity of the southern island mastermind. Likewise, the ninth volume hides the name of the student who leaves Class 3-E.
Minimalistic Cover Art: The volume cover art is usually nothing more than two solid colors and Koro-sensei's smiling face. Special mention goes to Volume 5, which resembles Koro-sensei's poker face: a blank cover and the ._. emote.
No Export for You: Eventually subverted. Initially, editors for the American version of Weekly Shonen Jump stated that, despite its popularity in Japan, the series will likely never be released stateside due to the subject matter. However, it has since been announced for a stateside release, but only in volume form.