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When you say "camera mounted flash" do you mean the pop-up flash, or a flash in the hot shoe?
If you're using a 580EX II on the camera, there's a known issue with the weather sealing on the flash foot sometimes causing the ETTL pins to lose contact with the ETTL disks on the hot shoe. When that happens, the flash will often fire at full power.
The solution is to wiggle the flash a bit as you're mounting it on the camera, and then push it into the shoe a little more firmly than you otherwise might while flipping the locking lever.
Recent 580EXs have redesigned pins that are less prone to not making contact. If yours are of different vintages, check the pins and use on the camera the one with pins that look like pointy rods, and the one(s) with pins that look like tiny popsicles for slaves.