I Deployed 6 AI Systems Live — Here's What Actually Broke

I Deployed 6 AI Systems Live — Here's What Actually Broke A few weeks ago I wrote about the 5 bugs that cost me 60+ hours building 49 AI systems. Every one of those bugs lived inside the code itself wrong array layout, a renamed model class, a serialization mismatch. This article is the second half of that story, and it taught me something more uncomfortable: code that runs perfectly on your machine can fail completely the moment it leaves your machine for reasons that have nothing to do with your code. I took 6 of my pinned GitHub projects and deployed every one of them live on Streamlit Cloud. Locally, all 6 worked without a single error. Deploying them surfaced 5 failures I had never seen before, none of which were bugs in my logic. Here they are, in the order I hit them. Failure 1 — A Module That Existed Yesterday, Gone Today My RAG chatbot used this import, unchanged for weeks: from langchain.chains import ConversationalRetrievalChain Locally: works. Deployed: instant crash. ModuleNotFoundError: No module named 'langchain.chains' The cause had nothing to do with my code. My local environment had an old, cached version of LangChain installed months ago. The deploy environment did a clean install and pulled whatever the latest version was at that moment and recent LangChain releases moved legacy chain classes like this one out of the core package entirely. The fix that actually worked pin the exact version that still contains the class, rather than chasing the newest API pattern under deployment pressure: langchain = =0.3.7 langchain-community = =0.3.7 The lesson: "it works on my machine" is frequently true specifically because your machine never reinstalled anything recently. A clean deploy environment has no such luxury it gets whatever is newest the moment it builds. Pin your versions before you ever need to debug this at 1 AM. Failure 2 — A File That Exists, Until It Doesn't My construction RAG project loads a prebuilt FAISS vector index from disk: vectorstor