The night before a race, I adjusted my rear derailleur. The race had a 2 mile hill that is 6% incline. Before the hill, I realized that my front derailleur wouldn't shift to the small chainring. I wasn't sure what to do, so I decided if I was going to climb that hill, I had to get a high cadence. I got to the hill, and I was the eighth wheel. I started pedaling as fast as I could in a 53/28.
Before I knew it, I was flying past everyone. I was screaming up the hill faster than any of the riders. I thought, wow, I've got ride this way on every hill. And then it happened, my heart was beating through the roof. I tried to hang on, but 3/4 up the hill I blew like the wind. I saw stars and thought I was going to faint. The group all passed me and several said, are you all right?
This is why pacing is so important. Also, don't try and adjust your gears before a big ride. Lesson learned.