One of the biggest mistakes I have seen in training design is biased one-sided training that emphasizes one component of training to the exclusion of others.
Effective training demands that all components of training be trained throughout the training year. Different components receive different emphasis depending on the time of the year, stage of career, competitive schedule. As I design my training I think of major emphasis and minor emphasis. A major emphasis will show up more frequently during a training cycle, a minor emphasis less frequently. As the athlete progress throughout their career the emphasis will change to meet the athlete’s needs and insure continual adaptation.