Bruno Druchen, national director of the Deaf Federation of South Africa, said the man "was moving his hands around but there was no meaning in what he used his hands for.
South African sign language covers all of the country's 11 official languages, but three sign language experts told the Associated Press that the man wasn't signing in South African or American sign languages.
"It was horrible, an absolute circus, really, really bad," said Nicole Du Toit, an official sign language expert who watched the broadcast. "Only he can understand those gestures."
Collins Chabane, one of South Africa's two presidency ministers, said the government is investigating the matter but has not finished yet because it has been overwhelmed with work preparing for Mandela's funeral Sunday in his hometown of Qunu.
The Associated Press contributed to this report.
Karen Workman, Digital First Media