I've got a similar situation to singindave's missed target gravity, but for the most part I'm hoping for some ideas as to what could have gone wrong (my yeast is already working happily, so I'm a little late to fix it now).
The other night I was brewing a stout with a target OG of 1.069, and wound up with an actual reading of 1.062. Going into the brewkettle I had a SG of 1.056 (after temperature correction), 1 point under my target of 1.057. My usual target volume before boiling is 7 gallons, but for this batch it was probably closer to 7.25 gallons.
Was it the extra volume that threw off my brew, or can you think of anything else that could affect it?