Waaayyyy back in the very early 1960s, I was a windowman at McDonalds; the early McDonalds had no seats (all walk-up service) and sold 15-cent burgers, 10-cent fries, and 20-cent shakes. In those days, McDonalds had a very simple menu, but if someone asked, we could make them a grilled cheese sandwich or double-meat burger.
Things have changed... and they haven't. There are many popular fast-food restaurants, and most of them will sell products that are not specifically shown on their menus. A link to the hidden menus is here: Hack the Menu. An indication of what might be available is in this article in the Daily Mail [via MA].
One of my favourites, which isn't on the list but which I construct myself (less often than once a year, to be sure), is the quad quarter-pounder from McDonalds: order two double quarter pounders and put 'em together (without the extra bun). Now that I've seen it, though, I may soon want to try the Mc10:35.
However, as much as I love these items, there is nothing that tops a Skor Blizzard from DQ, and that's on their regular menu.