Most species of snakes found in South Africa are indeed harmless to man. They either have no venom at all or have a mild venom that does not cause much harm to man.
An example of a mildly venomous snake is the Herald Snake pictured above. Also called the Red-Lipped Herald Snake as some look as if the have put red lipstick on. As a threat display they flatten their black heads, bulging out their cheeks and arch their necks. They strike readily. Although I have caught 100s of these snakes, I've never been bitten by one.
Those bitten get a throbbing headache.