The Tesco lottery is a game played by people who get deliveries of groceries from the supermarket chain Tescos.
Tescos seem to be remarkably bad at packing one's whole order in full and without bits of other people's orders. Thus, some deliveries you find you are missing a few items; you have lost the Tesco lottery, as some other person has your smoked salmon... however, you just call Tesco and get a refund.
Now, somewhere else there is a person with your smoked salmon and somebody else's bottle of vodka, but they are missing some toilet paper; they call Tesco and get a refund on the toilet paper, but keep the other stuff they didn't order; they have won the Tesco lottery.
The fun thing with the Tesco lottery is that the only real looser is Tesco.
1. "I won the Tesco lottery today... I didn't get a bottle of Pepsi, but I did get two bottles of wine, a pack of coco-pops and some rump steak in exchange; oh, and of course I called Tesco for a refund on the Pepsi."
2. "Damn it, I lost the Tesco lottery! Where's my wine, coco-pops and steak? All I have is this Pepsi I didn't order. Oh well, I'll keep the Pepsi and get a refund from Tesco on the missing stuff."