The Open Slot

When facing Red atop Mt. Silver at the end of Gen 2 (Pokemon Crystal), the sixth slot in AJ's party was open.

Lore Interpretation
Many popular interpretations of the "open slot" say that the spot was left open in honor of the fallen comrades of LazorGator and company. Others view the "open slot" as the aftermath of accidently depositing Ace, which it literally was.