A post I made earlier today reminded me of my own short booking window for one cruise. I was on a weekend cruise of SS SeaBreeze in 2000, which is my all-time favorite ship (tied with SS Norway). Given that we'd enjoyed the ship previously, and had a great time on that weekend return, we literally walked off the ship, called the travel agent, and booked again for the next month.
Unfortunately, the line went out of business just one day before we were to depart. Still, this is the first, and only, time we've ever done something like this.