I noticed it in the first batch I used Magnum which was a black IPA. I used quite a bit of other hops for flavor/aroma and dry hop. But a distinct flavor still comes through in the end. The next was a milk stout. The third, a chocolate stout. It's not as noticeable in that one, but I think it's because of the chocolate and orange peel additions but its still there. At that point, I didn't know yet what was causing that flavor.
After that, my next two batches didn't use magnum, a DIPA and a hoppy Belgian pale ale. That flavor isn't present in those two batches.
Then, I brewed a rye IPA using magnum at 60 minutes like all three of the other magnum batches. Took a hydrometer sample yesterday and there it was. That flavor I don't like. That's when it hit me that all four of the batches with this flavor used magnum for bittering.
Different grain bills, different yeasts, fresh ingredients, temp controlled fermentation. First batch was in October 2012, second and third were December 2012, and this fourth one was a couple weeks ago. I've never had this flavor in any other batches my six years of brewing before the first batch using magnum.