Promotions - Are they allowed to cancel early?

Recently I had a look at Dell's website and they were having a sale with about $200 discount on every system until the 29th August. 3 hours later (promotion ran from 4-7 am), the discounts were removed. I rang Dell up about this and they said that they had sold enough systems during that time and the promotion was not available anymore. I personally doubted the rep on the other end. Who buys a whole heap of computer systems at 4 in the morning? Does Dell have a right to pull promotions off like that, or was it more likely an error which they rectified? I personally have never seen sales from Dell being removed that fast.

 

Usually when they run a promotion its untill a certain date, or 1000 uses, and usually it's what ever comes first, not all promotions are like that, but I know dell and HP have both put out alot that are, Check http://www.cheapstingybargains.com/ for the latest deals for both hp and dell.