The $2,300 Weekend: When Fallback Routing Goes Wrong in AI Gateways

A single misconfigured fallback line turned a $40/month API bill into $2,300 in 48 hours. Here's what happened, why it's the most common LiteLLM mistake, and how to fix it before it happens to you.

What Happened

Last month, I set up LiteLLM Proxy to route traffic across multiple providers. My primary model was DeepSeek-V3 at $0.14/M tokens — cheap, fast, good enough for 90% of my traffic. As a fallback, I configured GPT-4o "just in case DeepSeek goes down."

Sounds reasonable, right? That's what I thought.

Friday night, DeepSeek started rate-limiting (429s). My fallback chain kicked in. Every single request that got a 429 rerouted to GPT-4o at $2.50/M input + $10/M output — 18x more expensive on input tokens alone, and over 70x on output**.