If an adventurer's cooldown track is full (can hold up to 13 dice) and they must add additional dice (such as from fatigue or status effects), the following steps are taken:

• If the track is full and a die must be placed in it:
  • That die must either be drained (if it is a skill die) or returned to the supply (if it is a fatigue or status die).
  • Then, replace the rightmost non-overfatigue die in the track with an overfatigue die.
    • The replaced die must also either be drained (if a skill die) or returned to the supply (if a fatigue or status die).

This process ensures that the cooldown track never exceeds its maximum capacity, and introduces overfatigue as a penalty for exceeding the limit.

Source: page 23

Source: Core Rulebook (pp. 23, 24, 45, 60, 89)