Where a provider offers a deal in the knowledge that they won't have adequate supply to meet the reasonably anticipated demand for such deal this constitues false advertising and deception. Further, in such situations a general statement along the lines of 'while stocks last' is not adequate.

In the advertising of any such deal a provider should include an indication of how long the deal is expected to be available for.

So for example,

UNACCEPTABLE: Specials from 1 April to 30 April: Tinned beans 2 for the price of 1 (while stocks last)

ACCEPTABLE: Specials from 1 April to 30 April: Tinned beans 2 for the price of 1 (while stocks last, it is expected that this deal will sell out by 3 April)