The unidentified man, who was on stage for hours alongside U.S. President Barack Obama and others, appears to know, at most, only a few words of American or South African Sign Language.
"There was absolutely nothing there," said Vera Goudie, a Toronto-area interpreter who learned American Sign Language to communicate with her parents, who are both deaf.
Observers say the man just signed the same few words over and over again — and quite poorly at that. His gestures came close to indicating words that included "running horse," "friend" and "beyond."
He also used almost no facial expressions — a must when signing — and did not spell out peoples' names.
South African Sign Language covers all of the country's 11 official languages, though it's customary for interpreters to sign in the language being spoken, which was English.
Goudie says the man’s presence was an affront to the deaf community and to Mandela.
"For a man who fought his entire life to help people break free of oppression, that oppression was happening through this man who was faking being an interpreter at his funeral service. It's disgusting," she said.
"Worldwide, deaf individuals are still struggling, are still oppressed," she said.
The South African government says it is investigating. The unidentified man was also spotted signing gibberish at an official event last year, prompting a complaint from the Deaf Federation of South Africa.
There were other signs of bad organization at the service Tuesday, including breakdowns in public transportation and a faulty audio system. Police also failed to search the first wave of crowds who rushed inside the stadium.
Bogus sign language interpreters are a problem in South Africa because people who know some signs, frequently because they have deaf relatives, try to pass themselves off as interpreters, according to Ingrid Parkin, principal of the St. Vincent School for the Deaf in Johannesburg.
Those contracting them usually don't know how to sign, so they have no idea that the people they are hiring cannot do the job, she said.