Other mistake: When Alfalfa takes some flowers to Darla, Waldo (the rich boy) turns up and Darla's friends comment on the rich kid. As one girl is saying her line, 'He's smoother than a baby's bottom' you can see the girl next to her mouthing the same line.
Other mistake: When Waldo and Darla are heading towards the go-cart race, as Waldo is phoning his dad, you can see that Darla is looking straight at the camera, then looks at the back of Waldo's head and keeps looking that way for the rest of the scene. It's obvious that Darla was not supposed to be looking at the camera, especially if you watch the ending credits, where people are telling her not to look at the camera.
Answer: I tried looking this up on the IMDb and couldn't find anything about Robin Williams there. My only suggestion is to watch it again and see if there is a list of extras in the credits somewhere. Maybe that's all the part was, an extra.