Not Knowing How to Change a Flat
While doing local race support and traveling with the last athlete in the race, I came upon a fellow that flatted one block from exiting T-1. I stopped to see if he needed help. Indeed he did. He held up a CO2 canister and said to me, "I can't figure out how this is going to put air in the tires."
One week before the race he purchased a gently-used time trial bike and it included a saddle bag with flat changing tools. That is, some flat changing tools. The bag had CO2 cartridges but no adapter.
Before your race, practice changing a flat tire with whatever you plan to carry on race day. Some people prefer CO2 cartridges over a pump because they normally make the process faster. But, if you don't know how to use them and all the CO2 shoots out into the atmosphere and not into your tube, all that time saving is lost.
More: 10 Bike Fix Essentials
Neglecting Your Bike Gears
Riding support behind the last racer at another race, I noticed the athlete wasn't changing gears on the hills. This person just stayed in the same gear all the time. I asked about changing gears and she told me she didn't like to change gears because she had trouble remembering which lever moved the bike into a harder or easier gear. She also told me she had trouble with the chain dropping off the front rings and decided staying in the same gear all the time was just less trouble.
Doing a triathlon on a single-speed is admirable, but much more work. The main benefit of using a gear appropriate for the terrain is that you optimize energy and time. Using a bigger gear than necessary on a climb slows you down and loads your legs with fatigue similar to weight lifting.
Assuming you're not training and racing on a perfectly flat course, use your gears often in training. Make multiple gear changes during the ride so your pedal revolutions per minute (rpm) are comfortable and around the 80 to 90 range. Some steep hills may drive lower rpm rates and some athletes might prefer higher rpm than others. In any case use your gears so the bike ride is less work. That saved energy will make your run faster.
More: Intro to Bike Gears
Celebrating a Podium Position Before the Race
A very strong cyclist I know was planning on doing his first triathlon. He carefully calculated and predicted his swim, bike and run paces. After reviewing the previous year's results, he confidently assigned himself a podium position in the weeks leading up to the race. The night before the race he stuffed his gullet with the pre-race pasta dinner and threw back a few brews. He and his two buddies were celebrating how easy this "triathlon thing" was for good cyclists.
Since he had high school swimming experience, he went out fast and hard on the swim on race day. Finished with the swim, he felt a bit queasy and dizzy getting on the bike. No matter, he put the throttle down on the 40K course treating the bike leg like the cycling time trials he had done in the past. At roughly the 30K mark on the bike course he began feeling bad. Really bad.
With his legs, stomach and head revolting he was forced to slow the pace. He knew he was losing time on the bike but he thought his lead was enough to hold off competitors. When the first competitor in his age group passed him with 5K left on the bike, his heart sank.