I've read of it happening, and I had a close call once. I had used some of my matches on a camping trip, with the strip of striker turned to face the case. Opened it up, and the striker was in the middle of them all. It must have shifted during the day. Nothing happened, but since that was my only thing of matches at the time, I consider it a close enough call.

The odds are low, super low, particularly if you don't do things like toss your pack off a cliff or anything so silly. I used the time bomb reference becuase becuase no matter how unlikely something is, if it can happen, it will if you wait long enough. While I have no problem with taking risks, if I can minimise them, I do so. Particularly if they are ones you didn't plan for. *shrugs* I hate suprises.