While most folks are most worried about not getting enough training, it is often the issue of overtraining that can be the most damaging to cyclists. Overtraining can result in a major, several week setback to your fitness. The balance a coach and athlete must play is between the fine line of overreaching and overtraining.
Here is a good article by triathlon legend Dave Scott.
I think the struggle for racers is that they refuse to believe they need the recovery necessary to get your legs back after overtraining. I’ve seen it a few times with folks I have coached, often in the early period we have worked together. This is the time where I am still learning about an athletes ability to handle work loads and recover. And it is often the time when a racer is uber motivated to train and can easily overdo it. I always encourage my racers to err on the side of recovery and to keep my up to date if they begin to feel any of these types of indicators for overtraining. If they do, it is usually a immediate shut down and rest approach, followed by test training days to see how we are coming back to full strength before we fully get into training again. I’d rather be slightly undertrained than have to take off months to get the legs back.