Let me clarify Error 99.
The camera posts that message to mean "Error, I don't know exactly what!"
What does that mean?
It could possibly mean about anything. It could mean that there was some intermittent problem inside the camera that came and went before it could really log it. However, in a majority of the cases, it means some kind of mismatch communications error either with the lens or in the lens mount. If any Canon or Sigma or any other brand has bad/intermittent communications between the lens and the body (around the gold contacts), then it is a problem. It could be a lens mount that is worn out and loose.
What does that mean?
It could be an older lens design that is not fully compatible with the Canon spec (which is not an open spec). Some of the old Sigmas were that way. Some older lenses can be brought up to the modern spec and then work. Some can't. Some lenses were sold so cheaply that the vendor can't rationalize spending the money to upgrade them, so the customer suffers.
My understanding is that it has to do with what happens when the lens goes wide open. There is supposed to be almost constant communications between the camera body and the lens, and when one (i.e. the lens) says something that the other (i.e. the body) can't understand, then the body posts Error 99. That means that the lens might work fine for a while, and then fail miserably for a while.
My personal opinion is that I would not have one. I would return it to the retail store.
If you got it from eBay, then that makes a case for doing business elsewhere.
---Bob Gross---