Forbidden Knowledge

• The Floors of Nevermore: Can only be discussed with those who have experienced it already. Jake mentions that Villy can't tell him about Nevermore because he literally would be unable to hear or see any information regarding the world wonder because he has not yet completed it. Later, when he meets up with other attendees from the 93rd Universe, they are able to discuss floors that they have all already completed.
• The First Sage: Initially, the Viper could discuss the First Sage with a small number of his close confidants, but by the time Jake is approaching peak C-grade, the First Sage appears to have become nearly system-wide forbidden knowledge with not even Duskleaf, Artemis, or even Eversmile being able to hear Villy when he speaks of the man. For reasons, not yet known, Jake, however, can still remember and discuss the man.
• Information about divine category skills: Mortals aren't even allowed to know about Divinity tier skills and abilities until they discover them for themselves. When Jake finally obtains his first Sacred meditation, Villy mentions that he tried to tell Jake about the skill before but Jake was unable to hear or remember the conversation. Later, when Casper asks about Jake's new skills, Jake can't even tell him about the Sacred category because even knowledge of the category is forbidden knowledge for those who have not yet obtained a skill of that rank.
• Information about how to reach divinity: When Jake obtains his first sacred skill, Villy jokingly begins to tell him about how to move from S-grade to god-tier. Jake can feel the System forcibly censoring the information, describing it as suddenly spacing out with no memory of why.

The Malefic Viper- Vilastromoz possesses the Bloodline of the Immortal Mind, which gives him a perfect memory, meaning he never forgets information even if it becomes forbidden. This ability to remember defy the System's censorship led to Vilastromoz being called "The Keeper of Forbidden Knowledge".