The most common mistake is picking a periodization model as an identity statement rather than a tool coaches attach themselves to ‘I run undulating’ or ‘I run conjugate’ as a philosophy, then force every athlete and every phase into that model regardless of fit. The meta-analytic evidence doesn’t support treating any single model as universally superior, so a rigid allegiance to one system is optimizing for the coach’s preference, not the athlete’s outcome. A second mistake is confusing variation with randomness genuine periodization varies load and volume according to a planned structure tied to a specific adaptation goal, while unplanned session to-session variety driven by boredom or a desire to seem sophisticated produces neither the stability of a fixed program nor the intended benefit of true periodized variation. A third mistake is under-crediting training age as the dominant variable: the same periodization decision that barely matters for a novice (who will adapt to almost any structured overload) can be the deciding factor for an advanced athlete chasing a much smaller remaining margin of improvement treating both populations with the same programming logic wastes the novice’s easy gains and under-serves the advanced athlete’s narrow window.