Error 99 appears using 17-85 lens only when some zoom is being used

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Err 99 could mean several different things, such as... Memory Card, Lens or Lens contacts....

Here's what you can try:
Remove the memory card an enable "shooting with out memory card" and see if you still get the error. If it works your memory card is bad. Try another lens if you can. Clean the lens contacts with a pencil eraser (be careful not to get eraser dust in the camera body or in the lens). Worst case scenario is, Err 99 could be related to the shutter failing which would mean a trip to Canon.
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Needs new zoom Flex, Tony

I hate to re-invent the wheel (and I don't want to cut 'n paste someone else's hard work to present as my own) so please click here for a likely practical solution which usually works and for Canon's own suggestions on the error 99 fault.

If this has assisted you then please rate my answer; if not then add a comment to provide feedback and I'll attempt to provide an alternative solution.

First, is the lens older than about five/six years old? If so then the electronics are almost certainly incompatible.

Error 99 is a generic code to say the camera isn't communicating with the lens. The first thing to try is thoroughly cleaning the contacts on the lens mounting flange, whilst doing so clean the contacts on the camera body as well. Don't use anything metallic to do this with.

If this fails then you either have a faulty lens or simply an incompatible one. By turning the lens slightly as if you're removing it, you can ensure that the contacts don't align correctly and then may be able to use the lens fully manually. Focus the old fashioned way and if there's an aperture ring you can set it to what you require: the viewfinder display may not show the correct exposure but will expose correctly when you press the shutter. It's a real bodge though and not something you'll be happy doing for very long.

Error 99 is a NONSPECIFIC error message. It is almost always associated with the LENS. I first got this error on my EOS 20D whei I tried to use an older Sigma zoom lens (7 years older than the camera). I don't use off-brand lenses with my Canons. Try removing the main battery first followed by the time & date battery which is hidden in the battery compartment beside the main battery. This will reset your camera. Good Luck!

The lens is for Minolta/Sony A mount and you are trying to use it on a canon body which has the contacts and mounting points in a different position so it isn't going to work as well as a lens with the correct mount for the body you are using if at all you will need to get the lens mount changed or an adapter built to allow the lens contacts to mate up in the correct order
will be cheaper to buy a camera body to fit the lens or a lens that fits the canon mount lens

i had the same problem and i found this guide online fixed my problem

1. Turn off the camera.
2. Remove the lens, battery, and CF card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert a fully charged battery, and turn on the camera.
5. Depress the shutter button as you would to take a picture.
Does the “ERR 99′ message appear? If it does, then the camera should be
serviced. If it does not, then please proceed to the next set of steps:
1. Turn off the camera.
2. Insert the CF card.
3. Turn on the camera.
4. Format the CF card in the camera.
5. Depress the shutter button as you would to take a picture.
Does the “ERR 99′ message appear? If so, then the CF card is the most
likely source of the issue. Try using a different card.
If the message does not appear, please complete the following steps:
1. Turn off the camera.
2. Clean the lens contacts by gently rubbing them with a pencil eraser
or soft cloth. Be careful that you do not let any debris fall into the
camera body.
3. Reattach your lens.
4. Turn on the camera.
5. Depress the shutter button as you would to take a picture.

You need to have your Sigma lens rechipped. Contact your local Sigma representative, they should be aware of this, and will be able to tell where to take the lens to be rechipped.

Error 99 is a general fatal error.
which includes the problems above mentioned by Q4SteveB but also many others.

It can be caused by a dicky shutter, problems with the CF card interface.

If cleaning and trying a different lens has not worked. Not definitive but as a rule of thumb...
If it letting you take one picture then giving error 99.. and the picture is saved to the card (but may just be black) this is most likely the shutter needs replacing..

If it takes a picture and gives error 99 but there is no picture on the card or the picture is 0KB its a CF interface problem.

If you get an error 99 when you plug in the USB its a USB interface error, you can try a different cable but unfortunately in most cases if another lens or cable wont work it needs repair

This is usually a torn flexible circuit or binding aperture in the lens. It'll have to go to a repair shop for replacement. About a $80 job.