I did my 4th triathlon over the weekend. My bike chain slipped off the ring during the car ride over and I didn't realize it until the bike leg of the race. This happened to me during my second tri as well. It probably cost me 30 seconds each time. Be smarter than me and check those chains
The good news is that I took 2 minutes off my course PR and got an AG award.
Not just checking your chain... you should always make sure you have your bike in a good starting gear before you rack it so you are ready to pedal as soon as you are on.