Lesson learned

Always, always, check both wheels before riding.  Got my new Cervelo P2 SL about a month ago and have been very happy.  Sprint tri on sunday for its first race.  Had the front wheel off to get it into the van and had it on nice and snug prior to the start.  Unfortunately, I didn't check the rear.  As I eased on the rear brake coming into the first big corner the wheel came out of the rear dropout and locked up completely!!  Thats when the fun began.  I fish tailed left, then right, then got launched over the aerobars landing on my back in the middle of the road.  Ouch.  The cassette was wedged against the frame and it took me several minutes to get it unstuck and get the wheel back on right.  Took a little of the starch out of me for the remaider of the bike and run but I finished anyway. 

Other than my suggestion above, my question for any biking guru's out there is why is the rear dropout on my Cervelo oriented directly rearward, parallel to the ground (3 o'clock) and not down and to the front like at the 7 o'clock position like the other bikes I've owned.  I figure there must be a reason but I don't think this would have happened by simply braking if it was oriented the other way.