aah, tesco discount bugs. my mums a fan of those, although they did fix the one she was most fond of.
say an item was £1.99, and it was BOGOF, the till would put it as
item = £1.99
item = £1.99
offers = BOGOF -£1.99
total = £1.99
what my mum did was to go to the reduced section in the store and get the BOGOFs from there, so the same item reduced to say 99p.
item = 99p
item = 99p
offers = BOGOF -£1.99
total = -£0.01
so tesco owed my mum 1p for buying 2 items
combine this with the monthly shopping resulted in 4 trollies full of stuff for about £20.
its probably a similair bug with the wine discounts. so only a matter of time before its traced and fixed, so stock up while you can.