Dark energy has a bit of a weird history. Initially Einstein included it (or rather something like it) in his equations for general relativity because it fitted

Dark energy has a bit of a weird history. Initially Einstein included it (or rather something like it) in his equations for general relativity because it fitted nicely and otherwise a static universe would eventually implode, only later it became clear that the universe was expanding and Einstein famously called this parameter his biggest mistake, except that in the current theory the value is very much not 0 (although annoyingly it's a lot closer to 0 than quantum physics says it ought to be, so much so that this has been called the worst prediction in physics). So yeah, the debate about dark energy is responsible for 'Einstein's biggest mistake' and 'the worst prediction in physics'. Either way it wasn't simply a case of "let's just give our error term a fancy name and call it a day".