It wasn't the major incident of the game and, ultimately, it didn't alter the result. Quite rightly, reports on the last South Africa v Australia Test focused on what a magnificent game and series it had been, and on the retirement of Graeme Smith, mentioning the non-dismissal of Vernon Philander only in passing. Yet it was an incident that was revealing, partly about the DRS but mainly about how people around the game think.

To recap: Mitchell Johnson bowled a horrible bouncer at Philander, who was struck on the shoulder, the ball looping up and being caught by Alex Doolan. Australia, believing the ball had hit glove before shoulder, appealed, and Aleem Dar gave Philander out. Philander reacted first by shaking his right hand vigorously and then, as though realising the implications of that, trying to pretend he was manipulating his shoulder - which, in fairness, had taken a meaty whack.

Recovering some composure, he reviewed the decision. The replay showed the ball had missed the left glove - his front one - and the bat handle, but that it had flicked the top of his right thumb.

The question then was whether the right hand was in contact with the bat. The thumb certainly was not, and neither was the palm, but the index finger may have been touching - the pictures were inclusive. If forced to decide one way or the other, I'd say the probability was that it was not in contact, but I can't say that with any certainty.

Now, normally of course, the benefit of the doubt goes with the batsmen, but the DRS requires conclusive evidence to overturn an on-field umpire's call. So that left the third umpire, Richard Illingworth, in a strange position: he knew Philander probably wasn't out, that there was sufficient doubt not to give him out if he'd been making the call fresh, but was the evidence that it was not out conclusive enough to overturn Dar's call? The not-out decision he ultimately gave - I think - was right in spirit, but probably wrong in law (which suggests a change in regulation is probably required).

Foolishly, I made an attempt to articulate that on Twitter, the 140-character limit of which, of course, makes it ideally suited for nuanced argument. The reaction was very odd. Some - including the former England batsman Mark Butcher and the former Liverpool and Germany footballer Didi Hamann - weighed in either in agreement or with constructive points. (Hamann's retirement from playing seems to consist almost entirely of watching cricket, with the occasional television appearance as a football pundit, when he tends to look vaguely surprised to find himself in a studio, before saying something original and interesting in a hybrid German-Scouse accent). This is one of the great joys of Twitter: you suddenly find yourself embroiled in what feels like a fragment of an unexpectedly good late-night panel show.

But the vast majority replied with partisan nonsense. Australians said it was clearly out. South Africans said it was clearly not out. And Indians attacked the whole principle of the DRS. I'm used to this in football. I go through phases of blocking for stupidity when fans of a particular team choose to see black as white or vice versa. I remember in particular an incident at Stamford Bridge when Chelsea's Fernando Torres fell in a slightly unusual way after being kicked hard on the knee by Newcastle's Vurnon Anita. From where I was sitting, the foul was obvious; from the first angle on television less so, but replays showed the kick clearly. Still, there were those with either a pro-Newcastle or an anti-Torres standpoint who refused to see what was manifest and insisted it was a dive. (Nothing, in fact, challenges my faith in democracy so much as Twitter.)

But what happened in Cape Town was weirder. This was people refusing to accept a grey area. The point I was trying to make was that the regulations need amending, yet suddenly by saying I couldn't tell from the footage whether the index finger was touching the bat or not, I became a despicable example of bias. "I don't know" had somehow become a highly contentious position, although the fact I was being attacked by both sides tended to prove the inclusiveness I was highlighting.

Does it matter? Not particularly. The DRS is still relatively young and eventually it will be refined into a model that doesn't leave the third umpire facing the sort of impossible challenge Illingworth faced on Wednesday. There probably will always be awkward instances that fall between regulations, when players, fans and journalists have to accept that out or not out, penalty or no penalty, are both acceptable decisions.

But the issue does indicate the tribal lunacy of some fans and the weird aggression with which that is often expressed. I've spoken often about sport's capacity to forge bonds of connection, and the value that has, and social media should be a major part of that. But blind partisanship isn't communication: sometimes people need to consider the issue rather than just who they want to win.