IN 1964, JUSTICE Potter Stewart uttered the most famous phase in the history of the US Supreme Court.

When attempting to define hardcore pornography in the high-profile case of Jacobellis v Ohio, Stewart said:

“I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description [hardcore pornography], and perhaps I could never succeed in intelligibly doing so.

“But I know it when I see it, and the motion picture involved in this case is not that.”

Equally hard to define, but you still know it when you see it, is the idea of coincidence.

Perhaps the best definition comes from mathematicians Persi Diaconis and Frederick Mosteller, who described the phenomenon as “a surprising concurrence of events, perceived as meaningfully related, but with no apparent causal connection.”

What their 1989 paper — Methods for Studying Coincidences — discovered is that people can be fairly liberal when it comes to describing coincidences.

For example, you might think it’s pretty coincidental to meet someone with the same birthday as yourself. But, with just 365 days in the year and seven billion people on the planet, is it really that much of a coincidence?

In case you were wondering, for there to be a 50/50 chance of someone sharing a birthday with you, there needs to be only 22 other people in the room.

That are times, however, when coincidences verge on the weird side. Take the case of the twins who were adopted by different families when they were four weeks old.

When they were later reunited for the above study, they discovered they had both been named James by their adoptive families, both were currently married to a woman named Betty having divorced a woman named Linda. One twin’s first son’s name was James Alan, the other’s was James Allan. And they both had adoptive brothers named Larry and pet dogs named Toy.

On Sunday night in Washington, the sporting world witnessed another weird coincidence as, 33 years to the day that Joe Theismann had his career-ending leg break, Alex Smith suffered an almost identical injury.

For many US sports fans, the image of Lawrence Taylor sacking Theismann at RFK Stadium is as fresh today as it was on that 1985 Monday Night Football broadcast.

It remains the most famous injury in US sporting history.

Don’t believe me? In 2002, ESPN polled its readers for the ‘10 most shocking moments in football history’ and they ranked the Taylor/Theismann sack as the biggest shocker… ahead of OJ Simpson’s arrest for murder.

The injury has featured in The Simpsons and is the opening scene of the movie, The Blind Side, as it signified a monumental shift in the importance NFL teams put on the left tackle position.

But the iconic nature of the Theismann’s injury is also due to the the fact, 33 years ago, long before Jason Witten was boring everyone to death, Monday Night Football made Americans drop everything and tune in.

So it’s probably fair to say, as Taylor landed on Theismann snapping two bones in the quarterback’s leg with one coming through his skin, it was the first time so many people had witnessed such a gruesome injury at the same time.

The broadcaster, ABC, showed the replay twice while Theismann was still on the field. Before the second showing, Frank Gifford said on air:

And again, we’ll look at it with the reverse angle, one more time, and I suggest, if your stomach is weak, you just don’t watch.”

And while it was immediately clear that Theismann’s season was done, what nobody watching that night knew was that, after staring 71 games in succession for Washington — winning Super Bowl XVII along the way — the quarterback had taken his last snap as a professional.

Unfortunately for ‘Hollywood Joe’, his right leg had healed in a way that meant it was shorter than his left. He couldn’t run in a straight line without veering. In July 1986, eight months after he took the hit from Taylor, Theismann failed a physical and was released by Washington.

Theismann was in the stadium on Sunday night as Alex Smith went down and immediately tweeted:

Of course, it’s far too early to call time on Alex Smith’s career. Medicine has advanced a great deal since 1985, but the 34-year-old is, at least, looking at a significant spell on the sidelines.

There are some schools of psychology that believe there are no such thing as random coincidence. For them, everything happens for a reason.

But according to our friends Diaconis and Mosteller and the Law of Truly Large Numbers, with a large enough sample, any outrageous thing is likely to happen. If they didn’t, nobody would win the Euromillions on a quick pick.

So while it may be tempting to look at 18 November as an inauspicious date for quarterbacks in Washington, the reality is that it’s just one of sport’s weird coincidences.

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