I had it happen just a month or two ago. It was dark and I couldn't find what had caused the flat. I replaced the tube and got maybe 30 yards before I flatted again.

After getting more light on the subject I found a sizable gash in my tire (that was probably bigger after the second blowout). I had to boot it to get home.

The other reason to carry a patch kit along with a spare tube is to be able to help out a fellow rider.