This is a thread for posting about sneaky things developers have done in or outside their games.

For example, I recently had to dig around in the ETS2 website for quite a while to find the store/downloads page for the non-steam edition of the game. Turns out, it in the SUPPORT PAGE! Yes, that's right, to buy the non-steam version of the game you have to go to the support page and scroll down a fair bit, rather than using the 'buy' tab at the top of the website.

To listen to Edith Piaf's "la goualante du pauvre jean", you have to explore the sound folders of NOLF's game CDs.

(Okay, technically it may require to unpack NOLF2.REZ and find SONG1.WAV. If I remember well, there is also an mp3 of Rina Ketty hidden somewhere...)

Incoming was a pretty awesome action game for its time. Perhaps it still is? Haven't played it ages. Anyway, this one time after finishing the game I decided to put the game disc into a cd player to see what would happen. All it did was to play all the game music, but when it came to the last track it started playing 'Old Macdonald had a farm' for some reason:
https://www.youtube.com/watch?v=h4-DmFSrY4w&feature=youtu.be&t=1836
This is still a mystery for me after all these years. I never encountered this anywhere in the game. Perhaps it's an easteregg that I missed?

Rogue Legacy has a hidden class that is only accessible by moving to the top of the screen where you buy upgrades and going up further.

In Super Mario Bros., in a couple stages (1-2 being one), if you walk across the top of the screen, you will reach a warp zone. (This is, of course, well known these days, but was novel at the time.)

In Metroid, if you go right from the start (as was standard at the time, since games back then typically didn't let you go left), you will reach a point where you are unable to progress because the path becomes too short to accommodate Samus Aran. Turns out that you need to go *left* from the start and get the Morph Ball in order to continue past that point.

Shovel Knight also has one stage where you can go left from the start, where you can see the game company's logo.

In Ultima 4, there is a shop with a hidden item. It may look like you can only buy a few items, but there is another hidden item you can get.

In Ultima 3, press "O" when asked for a new character's sex; this will make the character's sex "Other", allowing you to put non-binary characters into your party.

I once pushed on a portrait while playing Wolfenstein 3D and a secret room opened. Strange but true.

FAKE NEWS

Thanks, I didn't know about that.

Kirby's Adventure on the NES has a lot of unused content and debug rooms. But there is one things players can access, called the HAL Zone by despawning the warp star in Vegetable Valley 2. These rooms also exist in Nightmare in Dreamland but are inaccessible.

The link you posted doesn't work; it leads to an error page.

You can thank poor HTML processing for that one.

One of the GTA games had some leftover content in it. Something to do with coffee I think...

this is one is pretty well known by now, but back in the day I used to put all my PSX discs on a music player just to check if the soundtrack was in audio tracks format (loved the one from Real Bout Fatal Fury, couldn't stop listening to it), and when I put Symphony of the Night...

There it was, Alucard himself, telling me something like "this is a Playstation disc, you shouldn't play it on a music player"

♥

In Serious Sam you can kill the development team.

Pokemon puzzle challenge has a playable Panel de pon prototype level if you try to play it on a black and white gb and enter a code.

The Naboo Starfighter in Rogue Squadron wnd the codes for the Stop n Swop items in Banjo Kazooie are also probably good examples.

I can't believe nobody's mentioned Beamdog.