I really need to make sure I set my camera back to a baseline before I go out on any given day with it. Then that wouldn't happen (like getting all the noise). I'd tie a string on my finger but I'd probably forget why I did it......
I know...
My worst fear is forgetting to set the exposure correction back to 0. I practically never shoot manual, I let my 20D in 'P' mode and I have two dials to choose 1) the exposure/shutter ratio, 2) the exposure correction depending on subject and histogram info. The last one is 'sticky'... Fortunately, I shoot raw like you, and that has often saved my day! By the way, your shots only need more PP, they can yield excellent enlargements. The highlights are very clean, which proves exposing for highlights is still advisable, just like for slides.