The answer is ...when it is a so called large 500ml spray can of Scothgard sold on ebay.
It seems I buy everthing on ebay these days. This includes the majority of my carpet cleaning kit and like many I have stories of successful buys and great savings.
In the past I have bought 400ml cans of 3M Scotchgard off ebay. However wanting to makes savings I decided to try the other brands available. In total two other brands. Both were 500ml spray cans and about the same price of my 400ml 3M cans however the main attraction to me was the large discount should I buy 12 cans. I decided to "test the water" (apologies for the pun ! ) and buy a can of each.
My intention was to test the effectiveness of these brands using my Dupont fabric protector oil/water repellence test kit. Even before I reached for the test kit I realised there was a problem. My 3M can was heavier than these new so called 500ml brands and in fact one was so under weight I just had to laugh.
The results of the tests were interesting too. The 3M brand was more effective at repelling water born stains that the other two brand. All three products including the 3M were poor at repelling oils but I expected that, however 3M was again the best.
I know most CC will buy fabric protector in bulk and use a commercial sprayer but if your buying single tins them Ive found 3M the best.