Usually when people talk about Einsteins biggest blunder they talk about him not liking his gravitational constant - something that he had to incoorporate to make the expansion of the universe fit with his general relativity theory - and fit with what was already known about gravity today, the gravitational constant can be see as something that work against gravity. Today they again really need this constant to explain that the universe is in fact not only expanding but expanding acellerated!. Major interessting stuff