I was practacing some off camera flash work and was shooting after dusk. My ambient light reading was something like 1/60, f/1.0, ISO 4000. with a canon 580 ex in manual mode at 1/128 power, my light reading was something like 1/60, f/4.5, ISO 4000. However, my image was over exposed. I dropped the ISO on my camera down to 1600, then the image was properly exposed.
I was using a Sekonic L-308S as a light meter (and now understand why a backlit LCD screen is important, which that model lacks) that is usually right on. Why would my image be so over exposed?