Things like golf scores, the earth's temperature, and chronic back pain fluctuate naturally and usually regress toward the mean.
The logical flaw is to make predictions that expect exceptional results to continue as if they were average (see Representativeness heuristic).
Incidentally, some experiments have shown that people may develop a systematic bias for punishment and against reward because of reasoning analogous to this example of the regression fallacy.
For example: After the Western Allies invaded Normandy, creating a second major front, German control of Europe waned.
Clearly, the combination of the Western Allies and the USSR drove the Germans back.Fallacious evaluation: "Given that the counterattacks against Germany occurred only after they had conquered the greatest amount of territory under their control, regression toward the mean can explain the retreat of German forces from occupied territories as a purely random fluctuation that would have happened without any intervention on the part of the USSR or the Western Allies."
The reason is that political power and occupation of territories is not primarily determined by random events, making the concept of regression toward the mean inapplicable (on the large scale).