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same thing happened to me but it was because once when i took a lens off the mount something got bent inside the camera (not sure what to call it. silver level that moves aperture on the left side of opening). I would move lever up and down with my finger and it would reset to where it would open blades all the way so it wouldn't be dim. was good for two weeks then happened again. brought it in to be fixed and all is good.
does this make sense?
I was going to propose this as I was reading the thread: I think it's exactly what you experienced.
There's a physical lever moving the aperture lever on the lens (as opposed to sending an electrical signal to move it). The "resting" state of the lens is stopped down, so if the lever isn't catching, the lens is going to be stopped all the way down.
Taking another picture causes that lever to cycle and it's probably just bent enough to miss sometimes, but not all the time - so it catches, opens the aperture, and boom.
Very easy to see this: next time it happens, flip your camera around and look through the lens. If the aperture blades are anything other than wide open, then this is your problem.