90% Apparent Attenuation?

I brewed this Nut Brown ale last week and the recipe had the target OG = 1.054, FG = 1.015, IBUs = 18, Alcohol 5.3% by volume. My OG was 1.052 and when I transferred it to the secondary it was 1.005! Any ideas why the gravity would drop so rapidly? It was fermented at about 64 degrees for 6 days with normal activity.

6.5 lbs. light extract syrup
1 lb. dextrin malt
0.5 lb. Victory malt
6 oz. brown malt
4 oz. crystal malt (120 °L)
2 oz. black patent malt
2 oz. chocolate malt
5.8 AAU Willamette hops (bittering hop)
1 tsp. Irish moss
Wyeast 1056 (American Ale) yeast

Are you absolutely certain of the reading? It would be very difficult (I would imagine) to reach that attenuation without using a strain well suited to it. If you reading is correct the only thing that comes to mind (which hopefully didn't happen) is a wild yeast somehow got in there. They can supposedly eek out all but the last few gravity points. Has it dropped since then?

Positive on the reading. I have never had a drop like that before so I am fairly confused. I don't know if it has dropped since but I will check tonight. Could there be any ill affects from this? It tasted good, and I assume it will simply have a higher alcohol percent? Thanks for the imput, this place has helped alot in the past!

No, no ill effects that I know of. But you'll know if it keeps dropping. If not, I am really not sure. Perhaps there are others who have heard of this. The best I have ever done was to hit the high end of the yeast's range, and that was with a really fermentable wort. Yeah higher abv. The thing is, if it is a wild yeast that is capable of this it may just continue fermenting until you have pretty much nothing left lol. What you can do though if that is the case is try to arrest it by bottling and refridgerating. In that case I wouldn't even prime. Just let it sit for a week and test the carbonation...