.. Prove at the time of the coining of that old cliché had the meaning test. The rule could be proved and found wanting.
While, at first sight, this explanation has a seductive quality, I am still unconvinced.
First, the corresponding german phrase has the word "bestätigt.." for "proves", and there is no way of interpreting this as "test", it clearly means "confirms".
Second, and more important, your hypothesis would mean that any rule which has a single exception is invalid and is to be discarded! this would, in my opinion, deprive the word rule of its specific meaning: "as a rule, I only cross the street when the light is green...".