The beta acids colupulone, lupulone and adlupulone can be auto oxidised to cohulupone, hulupone and adlupulone respectively which can contribute significantly to bitterness in beer.
It is stated that this is more an issue with storage and occurs in old hops, which most brewers would not use one would imagine, but with the cost of hops at the moment perhaps corners are being cut and this process allows older hops to be used.