But attributing the result to “bad luck”—and stopping there—misses an important point.
In observational studies, we call this imbalance confounding.
In randomized trials, the imbalance occurs by chance so we can call it "random confounding."
That is, in randomized trials, we are better off ADJUSTING for prognostic factors that happen to be imbalanced between groups.
More details in Chapter 10 of our Causal Inference book: hsph.harvard.edu/miguel-hernan/…