Website bug; cheaper flights oddity

Trying to book some flights from London to Jo'berg during December, 6th to 13th to be precise.

On these dates if I select Economy or Premium Economy flights it will show me a price in the matrix but then when I go to confirm the price it informs me my fare is no longer available and offer a CHEAPER price.

Bizarre! Not complaining but is this common?

I think there was a problem on one of the GDS last night and it had the wrong fuel surcharges loaded. I was looking at flights to Tokyo and they were about £60 cheaper than they should have been.
It's been fixed this morning though.
Ed