MUDs are very classic. In fact, they are played using Telnet, probably the earliest network protocol available. In a way, they are big text adventures with few restrictions, most notoriously to not double login, that is, players are not allowed to have two characters logged in at the same time. Much like MMORPGs nowadays, they divide the characterbase into different guilds.
A character belonging to a guild has access to certain skills, which vary depending on the guild theme. Spellcasting guilds usually require ingredients and special words to execute a conjure, which are jealously kept hidden. The player is forbidden to reveal that information to other players. Now, the player may have another character, a second, in another guild.
If both guilds were in war, the player would knew the weak points of each guild, which items to collect to protect himself and where to attack. Would it be fair for him to participate in the war at all, since he is relying in insider information? Could he suggest members of his guild to wear determined items? Could he even wear them without being considered a cheater?
Another example. Let's suppose the player has been playing with a warrior character for years and decides to create a spellcaster. The player already knows which areas are safe for him to travel at its current size, how to finish quests, where to go to join determined guilds, etc. If the player were to share that information with another player, it would be considered cheating. However, it is fine for him to reuse that information with his own new characters. In this case, it is accepted that the player cannot keep the knowledge of each character separated in his brain.
A final and more extreme example. Several characters are planning on how to catch their main enemy, and are preparing a trap. This trap would use many different items, rely on determined places and times, making it virtually impossible for their enemy to escape.
They decide to set it up in a room the enemy crosses every day, as they have verified it observing the location for over a month. But one of those listening is actually the second of their enemy or, in other words, is being controlled by the same player that controls their enemy.
What should this player do? If he doesn't enter that room where he will surely die, he would be cheating because he would be using insider knowledge to evade the trap. And even though the player would be the only one knowing he has cheated, the planners would guess there is a traitor who spoke to their enemy about the trap, even though technically both characters have never known each other (as the player cannot login both at the same time). Should he just enter there and hope for the best?
When knowledge becomes cheating?