There's nothing really bad about eating leftovers as long as the leftover food has remained refrigerated and is still good to eat.
Leftover foods can last for up to 7 days for most foods in the refrigerator and when frozen leftovers can last from 6 months to a year and sometimes longer depending on the food type.
I've eaten leftover food nearly all my life and nothing bad has happened to me.
If the food smells bad or has a bad taste to it or has mold in the leftover food them throw it out.
It's best to eat most leftovers within a few days but some leftover foods can last for 6 to 7 days in the refrigerator and up to 6 months to a year in the freezer.
The only thing that could be bad about eating leftovers is that bacteria can grow in the food if it's not promptly refrigerated or frozen.
Otherwise the leftover food is safe to eat.