I purchased the SX20 largely for the AUTO function, having sold my trusty XT2.
The rear control knob - without a lock button - is very easy to mistakenly turn and over-expose. This is very easy to miss as the scale on the left in the display is very small. Also when switching on and off it retains the over-exposure - why not reset to zero when switching off????
This I found to my cost on a trip around Cape Town in SA when the whole day was shot over-exposed - really really frustrating!!!!