My worst bite was not a feeding error, but an error on my behalf nonetheless. It was with a 4' male iguana. He had been acting a little funny one day and when I was in the bottom of his enclosure diligently scrubbing a mess he had made. He climbed down to a shelf that was right at my head level, i looked up and WHAM... he nailed my cheek. i swatted him off and he went a few feet, knocked over the bucket of dirty poopy water and then i unleashed a string of profanities not due to the bite, but due to the icky mess on the floor.
end result was a very territorial iggy, me with a flap of skin hanging off my cheek that required 4 stitches to close, and a Dr. at the hospital that phoned a vet to inquire as to how to treat me.
I suppose it was the location of the bite, along with my high pain tolerance, I would give this bite a pain factor of 5 or 6..