Imagine an online store, which displays the following message at checkout:
You may have been selected for a special offer! If you see a discount code below, type it in the "Have a coupon? Enter it here:" box to save 20% off today's order.
(If no code is visible, sorry, you are not a winner.)
The actual code would be randomly generated, expire after 20 minutes, and only be accepted if submitted from the same IP address (so it can't be shared with anyone else). However, everyone gets a valid code. It's just hidden behind a <blink> tag, which you can't see in Firefox 23 and later (and some other browsers). People who use those would be tricked into paying more for the same stuff. ;-)