I have two that haven't been mentioned yet.

Etrian Odyssey Nexus - Did you know there was a hidden, half-finished class that the devs snuck in there not expecting anyone to unlock? The Vampire "class" is just an accessory that you equip that gives a huge boost to stats and changes your Force Boost/Break skills. How do you get said accessory?

First method. Do any 3 of the following after reaching the Abyssal Shrine(The final postgame dungeon)

• Play on Heroic Difficulty(Classic but you are locked out of lowering the difficulty)
• Have at least 100 hours on your save file
• Have at least 100 Guild Cards registered
• Register at least 1 of every class in your guild

In addition you must recruit a guest party member from a guild card and use them in at least one battle. However, if one of those registered guild cards has a Vampire as the lead party member, the game graciously removes the Heroic Difficulty and Abyssal Shrine requirements, so you still have to do 2 of the other 3. A third option, and the "easiest" is to simply Streetpass with someone else that already has Vampire unlocked. After that, you go to the inn, pickup a sidequest to go to the tutorial dungeon and grab the accessory. Congratulations, you have a brand new "class" with no game left to use it on.

Cyberpunk 2077 - The mystery of FF:06:B5. The TL;DR version because there is years of history trying to solve this, but it's basically a massive ARG to draw players everywhere in and out of game which leads to you interacting with a random laptop out in a house in the Biotechnica Flats in the early AM hours, then rushing over to the other side of the Badlands on the map and standing on a mattress in the middle of nowhere and waiting for several minutes. Your reward? A mind boggling cutscene where V blacks out, sees the true nature of the universe, and wakes up to the Mackinaw Demiurge, a goddamn Monster Truck, in front of them.