Legal position on Easter Eggs and references in games?

I'm interested in whether game developers who have references or Easter Eggs related to other games in their games have to respect any copyrights. If both games were made by one person / company, it is probably clear, but if a single programmer from a Nintendo game wanted to install an Easter egg, would that just work? Hope you can answer the question for me. XD

That depends on the publisher. Some see it as an advertisement for their product, others have a stick in the *****.

Nintendo is there, for example. Very restrictive. They will tolerate a verbal swipe / allusion to Mario, but a character who looks like Mario in your game will sue you for copyright infringement as soon as you notice it. Already happened.

If in doubt, write to and ask, then you are on the safe side.