Gamer sites on the early Internet were full of these "Easter eggs" that were really just non-obvious things with clear explanations in the manual.

One that I found particularly irrimusing (and seems to keep popping up forever) was that holding some combination of buttons on the Gameboy Advance when you turn it on "plays a secret, alternative start-up sound, then it just sits at the Gameboy logo until you press a button. That's all it does."

Except if you read the manual you'd know that holding that button combo overrides the normal start-up and forces the GBA into multi-play download mode, so you can play those games without having to take the cartridge out of the console. Pressing a button in that mode cancels it and resumes normal start-up, loading a game from cartridge if one is inserted.

I've seen some people insist that their manual didn't say anything about this, but I have trouble believing them given that it was written in the manual for the GBA which I bought on launch day.