First off, my equipment and the problem:

• Canon Rebel XTi (EOS 400D) body, bought from Aden Camera on 2nd December 2007
  
• Sigma 18-200mm F3.5-6.3 DC OS lens bought at the same time
  
• Canon EF 50mm f1.4 USM lens, bought at Aden on 5th September 2008

For the past few days, I've been getting an error when I try to take a photo:

Error 99
Shooting is not possible. Turn the power switch to OFF and ON again or re-install the battery.

I tried turning it off and on; same error.
I removed and reinstalled the battery; same problem.
Because it usually have the Canon BG-E3 battery grip installed (with two Canon NB-2LH batteries) so I removed the grip and put just a single battery in the body; same problem.
I swapped lenses and the error went away temporarily; after a few shots, Error 99 was back.
I even tried changing the memory card (I use 2Gb and 4Gb SanDisk Extreme III CF cards) but the error persisted.

A couple of friends suggested using a pencil eraser to clean the connections between the body and the lens. I tried this (holding each component so that any dirt fell away from, rather than in to, the body or lens) and it temporarily fixed the error, but again a few shots later Error 99 returned.

So now I'm checking out Google to see what other people recommend....

One post says that Err 99 "is non-specific. The only way to find out what it means is to send the camera in for repair. Once the camera is hooked up to the service department computer, their diagnostic software can get more detailed information to aid the technician in solving the problem."

Another post has some steps to follow:

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.
   (Charging battery...)
   
4. Insert a fully charged battery, and turn on the camera.
   
5. Depress the shutter button as you would to take a picture.
   (I had to change my setting as I usually have "Shoot w/o card" set to Off)
   Took a photo OK!
   

One battery is still charging - it's taking a long time considering I only charged them a few days ago - I wonder if the battery is dying?

I've put the CF card in and it takes OK.
(Remembered to change "Shoot w/o card" back to Off)

Put the lens back on and again, takes OK.
Swapped the 18-200 lens for the 50mm; still taking OK

The other battery is now fully charged; swap batteries; still OK

Reattached the battery grip and inserted both batteries; still OK

One thing left to do: swap back to the 18-200 lens ... it's still OK
This means I'm back to the same setup that I had when I was getting Error 99 and now I'm not ... and the only thing that's really changed is that I recharged one of the batteries!?!

This is the most annoying outcome - as much as I'd hate to be without my camera for a few weeks while it's being fixed, this is worse because the problem has gone away without me being able to identify why ... which means when the error comes back, I'm no closer to knowing how to fix it :(

Update @ 2pm: I just tried the camera again and Error 99 is back! The odd thing is that the battery I charged earlier looks to have lost a lot of its charge! It might not have been the same battery.

Anyway, I removed the battery grip and used just a single battery, and I'm still getting Error 99.

Time to find out where to take it :(

Registered with the End-user Online Repairs site; entered problem info; no estimate ("Upon arrival of your repair, you will receive an estimate"); available service centre:

1400 Courtneypark Drive East
Mississauga ON L5T1H1
(905)795-2005

[map]

I can get a ZipCar and drop it off, which should save time/$$ in postage.

• You will receive an email confirmation within 48 hours of arrival, indicating your package has arrived.
  
• Repair completion is estimated to be 15-20 business days from email notification. Actual times may vary due to seasonal volumes, repair requiring special parts, or lengthy testing for unusual / intermittent problems
  

Guess that's all I can do for now :(