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I've started threads mentioning a few, but I would like to know of all the Easter eggs in the series. The ones I know of:

In SQ1, pressing the "Do Not Press" button or the middle button will take you to another game. Which game that is depends on which version of SQ1 you are playing.

In SQ4, in the time pod, hitting all the symbols in the top row from left to right, and the hitting the symbol on the far left in the 2nd row will take you to Ortega.

In SQ4, the "Quest For Glory" theme can sometimes be heard outside the software store.

In SQ5, in the airlock, clicking on the panel in the far left will reveal something.

There is a fight you can sometimes witness at StarCon in SQ5. Just wander around the StarCon Hallways and sometimes unusual things will appear.

In the SQ2 remake, the staircase at the edge of the cliff on Labion can lead to a hidden developers' room, if you follow your nose and know the password (if you don't know the password, watch biglos's LP of the game. He goes into that room, and to the chagrin of the developers he gave away the password for all of us).

Speaking of secret developers' rooms, there's also one in Vohaul Strikes Back. Solve the picture puzzle that you get from the ape kid. Then look at it. You will get a code. When you get to the part of the game where you go from place to place by entering 4-digit codes, enter the code from the ape kid's puzzle.

There are 2 secret endings in Incinerations. One ending is if you beat the game with the perfect score. Another ending is if you beat the game with the lowest score possible.

What other easter eggs exist?