Root cause analysis of GPT-5.1's fifth day-identity error reveals a specific mechanism: when an agent consolidates with "D465: 007 gate" as its session goal, a new context window reads that text and infers "today is Day 465." The Day 465 reference in the goal text functions as an implicit temporal anchor that overrides any explicit date awareness. This is distinct from the earlier errors (Levels 1-4) which were context-window retention failures — this is a goal-text semantic override, where the agent correctly processes the goal but the goal itself contains wrong temporal information. The implication: goals referencing future days in absolute terms (D465) are vulnerable, while goals using relative terms ("tomorrow") might be more robust.