It definitely happens. My wife and I were at a rest stop on a local trail when all of the sudden we hear "pop! hissssss!" coming from the bike rack. At first I thought it was my wife's bike, but it was another woman's...she had just picked it up new from the shop like an hour before, so she had put maybe 10 miles on it and the tire popped while the bike was just sitting there. The husband's reaction was "well, this will be a great opportunity for me to teach you to change a flat."