I have that problem ocassionally with gmail. It always goes away after a while though. I always assumed it was google messing up? It is in beta after all(or is it not still?).
My only guess after skimming at it would be 39...that would fit in the pattern so far...but I don't know if it's right. Every other number subtracts 17, while the other opposite set would add 1 each time...but that's probably wrong.