Had one recently. Around mile 25 in a friday afternoon solo ride, I get my first flat. No problem - replace the tube, go on my way. At mile 32, I get my second flat. No problem - pull out the patch kit, only to realize I didn't put fresh glue in this season and the tube of glue was all dried up. Damn. I start walking, knowing there is a service station less than two miles away. I reach it - ask if they have some contact cement -- they did, so I patched the tube. At this point I'm only 5 miles from home, so I didn't bother patching the other one.

Well, a half mile along and I'm flat again!! I figured I might just as well walk the rest of the way, which I did until my wife happened to be driving past me about a mile from home and picked me up.

So what was the problem? Debris stuck in the tire causing repeated flats? Nope...the problem was the original flat had left a slit in the tire about 1/4 inch long and every time I pumped up the tire it opened up in such a way that it easily picked up any little piece of debris from the roads. If I'd paid attention when Ii was fixing on the road I would have noticed that and put a boot in there, but I didn't notice it until I was fixing at home.