Why do you think I nearly fell on this turn? I was riding my new Scott road bike yesterday and I nearly took a terrible fall.  The pavement was wet and there was a curve.  I was riding approximately 15 mph and I braked with the rear brake.  As I applied the brake the rear tire nearly spun out from under the bike.  Do you know what I did wrong?
You braked too hard given the road conditions, plain and simple.

The road was wet, which decreases your traction. You hit the brake, which uses the tire's traction on the road to slow the bike. If pressed too hard, the rear wheel will lock up and skid, which can be disastrous in a corner — the rear wheel will continue to follow its outward momentum. 15mph may not seem fast, but that particular corner may have been especially dangerous for any number of reasons:


wet leaves are notorious for being slippery
automotive oil collects on the road if it hasn't rained in awhile, or if there's poor drainage
there was a dip in the road
the road may have been off-camber (the road slopes down toward the outside of the turn)


To avoid accidents in the future, the first thing you should do is properly learn to use your front brake. The front wheel will not skid under reasonable cycling conditions. Other than that, you'll over time develop a sixth sense of how safe the road conditions are. Just pay attention to the corners and watch out for dips, debris, puddles, and the camber of the turn.