Diacetyl is detectable in beer at concentrations as low as fifty parts per billion. At low levels, it gives beer a slick mouthfeel; at higher levels, the flavor becomes buttery, then like butterscotch, and eventually rancid. One possible source of diacetyl is bacterial infection. Pediococcus and Lactobacillus are notorious producers of diacetyl. If you can detect other off flavours like sourness, rope, or other odd aromas, infection is probably your problem.
As brewtopia said, no harmful bacteria can survive so if it doesn't taste half bad, drink it up! It might be interesting to do an experiment and save a few bottles for a couple of months. If the bacteria are responsible for the diacetyl, they will probably produce more of it, and you may be able to detect a flavor difference. If you do notice a difference, then at least you will know what happened.