Good catch. Outcome (or error, which boils down to the same thing) can't just be anything either, though, e.g. a common pattern is that the variance in an outco

Good catch. Outcome (or error, which boils down to the same thing) can't just be anything either, though, e.g. a common pattern is that the variance in an outcome increases when the predictor increases (e.g. a machine behaves more erratically when it spins faster) and that can mess with estimates.