Just a quick note to observe the simple solution to the "Err 99" message occasionally obtained with numerous Canon digital cameras - whilst this is not necessarily the cause, frequently, the problem is that the gold contacts are dirty. Solution? Easy: erase them. With a standard pencil eraser. At your own risk!

Other techniques which can work (i.e. try at your own peril) include:

1. Re-inserting the CF card
2. Re-inserting the battery pack
3. Re-aligning the shutter curtain (lift mirror and brush lightly into place)
4. Being careful about switching the camera off before changing lenses etc.
5. Letting battery go flat whilst exposed (?!)

Worked for me!

(Source: richardnotes.org)