Last week I purchased over £100 of goods from B&Q. As I joined their new loyalty club a couple of weeks before, I used a "get £5 off a £50 shop" voucher when I made the purchase.
Today I returned one of the items, costing £23, but they attempted to refund me around £21. When I queried this, they said that because I'd part paid for the shop with a voucher, they weren't refunding a proportion of the voucher.
I argued that the voucher was a fixed amount, not a percentage of the voucher, and that as they didn't give me £10 off the original shop and because the items I'd not returned cost more than £50, surely I should be refunded the full amount. They backed down and refunded me, but only because I queried it. I imagine people refusing more than one item may not notice that they are not receiving a full refund.