That doesn't have anything to do with how you stored your hard-boiled eggs, it is actually an indicator that the egg was ...

But what if you overcook a hard-boiled egg? You know, when the yolk is so overdone that it resembles a piece of Donald Trump's face crumbling into the sea. Do you prefer hard of soft-boiled eggs?

it’s actually safer to eat overcooked eggs rather than undercooked eggs, which can harbour bacteria.” Though the egg might technically be a little safer, most boiled eggs are good to eat.

These combine together when boiled, and if the egg is boiled at ... “The ferrous sulfide that’s formed when the egg is overcooked is actually the same form of iron you find in iron supplements ...