Wheels within wheels

Hidden or unknown things that influence a particular situation, making it more complicated than it at first seems.

Cambridge Dictionary

Can of worms
informal

A situation that causes a lot of problems for you when you start to deal with it:

Corruption is a serious problem, but nobody has yet been willing to open up that can of worms.

Cambridge Dictionary

Mailman Steals From Birthday Cards

9 dec. 2019

I don’t want to get too technical, but the phrase does go “Finders, keepers; Losers, weepers”.