
In that case:
1. The Atari landfill in New Mexico, which in fact turned out to be true after years of speculation.
2. All the various Mortal Kombst conspiracies that floated around primitive online message boards and game magazine letter sections in the series' early years, such as Noob Saibot and Ermac, as well as theories on how to play as Sonya and Kano in MKII. Ed Boon and John Tobias ended up bringing most of these to life, since Mortal Kombat is one of the most fan feedback-driven game series out there.
3. Product shortages of Nintendo products such as the Wii, Switch, or the NES and SNES Classics, and early on, the DRAM shortage of 1988 which a lot of people later speculated was Nintendo's way of drumming up extra demand for Zelda II. Made-up shortages are a running theme with regards to Nintendo, as a way to inflate demand through perceived scarcity. Honestly, I'm not sure that they're all that off-base sometimes, because Nintendo is proud of the fact that its products hold their value for years instead of ending up in the bargain bin for 20 dollars within 6 months.