Originally Posted by Bobby Lex
+1.
This is a very common cause of repeat-flat occurences. Before installing a new tube, run your fingers along the inside of your tire to feel for any sharp object that might be causing the flat. Sometimes the culprit is very small, and only pokes out enough to pierce the tube when the tire flexes.
Bob
You can turn the tire inside out to help expose the culprit.
I've gotten tube flats by yanking the air nozzle off the valve too hard. It makes a hole at the base of the valve on the tube, not a good place to patch.