Every one of these failures bit me while building n8n workflows with LLM nodes. They all pass in the editor and break under real traffic. Here is what breaks, how to reproduce it in five minutes, and the node-level fix.
1. The LLM returns prose, not JSON
Symptom: your parse step works on the test prompt, then throws Unexpected token in production.
Why: the model politely prefixes "Sure, here is the JSON:".
Test: feed it 20 messy inputs - typos, emojis, very short strings.
Fix: require JSON output where the node supports it, keep an explicit schema in the prompt, and add a fallback branch that retries once with a stricter prompt.
2. Silent API failure
Symptom: the run shows green but the downstream data is empty.
Why: plenty of APIs return HTTP 200 with an error body (rate limits, quota).
Test: mock a 429/500 and watch what the workflow actually does.
Fix: turn "Never Error" off, inspect the response body in an IF node, and wire an error branch to a notification.
3. Duplicate runs
Symptom: the same lead gets three emails.
Why: retries plus at-least-once delivery on webhook triggers.
Test: POST the same payload three times.
Fix: an idempotency key - hash the payload, store it, skip if you have seen it.
4. Empty and edge inputs
Symptom: one blank field crashes the whole run.
Why: only the happy path was ever tested.
Test: run with the minimum possible input (no name, no email).
Fix: a validation IF before the AI node, routing bad rows to a "needs review" bucket instead of failing.
5. It works - until the key expires or the limit hits
Symptom: 2am alerts because the workflow died silently overnight.
Test: none needed. Assume it will happen.
Fix: an n8n Error Workflow that pings you on any failure, plus retry with backoff on the API nodes.
The common thread: every one of these is a failure path nobody tested. We demo the happy path; production only fails at the edges.
If you want a working starting point, my free AI Hook Generator is an official n8n template - import it and read the sticky notes for how the prompt and error handling are structured:
- Official n8n template (one-click import): https://n8n.io/workflows/20418-generate-social-content-hooks-with-openai-gpt-4o-mini/
- Free workflow + source (MIT): https://github.com/zhp910318/n8n-ai-automation-lab
- Free download + the paid workflows it pairs with: https://payhip.com/haopengailab
And if you would rather not test this yourself: I run fixed-price pre-demo stress-tests of n8n AI agents - you export the workflow, I return a prioritized failure list plus patches.
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