"What's the best exposure for a candle-lit couple sitting in the moonlight on a darkened patio? How about for the best sunset of your vacation? Or, for bright shafts of sunlight filtering through palm fonds onto your subject? In tricky lighting situations like these, using your camera's built-in light meter to take a reading off your subject will often not produce the results you want. Bracketing your shot will be a good idea, but is still no guarantee. Likewise, you could use your flash, but to really capture the mood or the subtle play of light, you may not want to."
How many of you use grey cards when shooting? I haven't - I tend to rely heavily on post-processing the fix exposure issues, which of course isn't the best way to do it - it's better to get the photo you want at the source, because you'll have more to work with.