Scotland's Stuart Hogg has questioned why the Television Match Official (TMO) did not bring Siya Kolisi's apparent head-butt on Peter Horne to the referee's attention at Murrayfield.
The South Africa captain escaped with an official warning after World Rugby's citing commissioner deemed the incident to be "just short" of a red card.
The incident occurred in the 31st minute of the Springboks' 26-20 win.
"These things happen but you question what the TMO is doing," said Hogg.
"He can have a look three or four times, or as many times as he wants. The unfortunate thing was we got a good outcome on the back of that phase of play, so they didn't look back at it.
"Peter Horne is a hard bugger and just managed to get on with it. He didn't whinge about it.
"I asked Pete 'was that a legit head-butt?' and he said he felt he was head-butted. But it has happened. We can't do anything about it now."
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New Zealander Ben Skeen was the TMO during Saturday's match, with French referee Romain Poite missing the incident at the time.
World Rugby say mitigating factors were behind the decision not to cite Kolisi, including Horne "illegally preventing Kolisi rejoining play" and the "moderate force of the strike to the head".
The warning will form part of the flanker's disciplinary record, but he is free to lead his team against Wales on Saturday.
"I am just glad he is available for us, I am glad the guy who was involved on the other side is okay and that it has been sorted out so quickly in the week so we can move on," said South Africa coach Rassie Erasmus.
'Severe lack of consistency'
Asked if it was a concern that no action had been taken against Kolisi, given World Rugby's increased focus on protecting player's heads, Glasgow full-back Hogg added: "Player welfare is massive. World Rugby are trying to make sure we are in the safest possible place.
"I just think there is a severe lack of consistency at all levels of the game now. World Rugby will look into it. They are in charge."
Hogg insisted he will be ready to face Argentina on Saturday in Scotland's final autumn Test, despite being forced off in the 63rd minute against South Africa.
"I just rolled my bad ankle," he said. "I got caught in a tackle. It was a bit achey but nothing I have not had before.
"I tried to hide for a little while but unfortunately they kicked the ball straight to me.
"But it feels grand today and I am ready to crack on with this week. I woke up feeling a lot better than I did yesterday."
Hogg only returned to action against Fiji on 10 November, seven weeks after having surgery on the same ankle.