Ah, nothing like dynamically generated sub-optimal routing.
Smirk had me install a new router for our T1 customers, and the resulting changes in network topology were rather amusing. The expected path for traffic to customer R was “Internet → Core → Edge-02 (the new router) → R” but instead it went “Internet → Core → Core-02 → Customer S → Edge-02 (over Customer S' T1 no less!) → R.”
Ah, it's the multiple paths between the two cores that lower the “cost” for traffic. The fix was easy enough—bump up the “cost” of Customer S' T1 such that traffic will prefer a “cheaper” path and not go over Customer S' T1.