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Why Does Code Work Locally but Fail in Production?

Leader ●4 ●8 ●22
calendar_today • schedule1 min read

One of the most frustrating problems in software development is when everything works perfectly on your machine, but breaks after deployment. 💀

The tricky part? The code itself isn't always the problem.

Here are five things I would investigate first:

  1. Environment variables
    A missing API key, incorrect URL, or improperly configured environment variable can break an otherwise working application.

  2. Dependency differences
    Different package versions or runtime environments can produce unexpected behavior.

  3. API and CORS configuration
    An endpoint that works locally might fail in production because of origin restrictions, incorrect URLs, or deployment configuration.

  4. Database permissions
    A feature may work with local development permissions but fail when production security policies are enforced.

  5. Build and deployment settings
    Incorrect routes, missing assets, case-sensitive file paths, and build configuration differences can cause issues that never appeared locally.

My rule is simple: don't assume the deployment is broken until you've identified where the failure actually occurs.

Reproduce the issue, inspect the logs, isolate the failing layer, and test the fix before deploying again.

💬 Question for developers: What's the most confusing production bug you've encountered, and what turned out to be the actual cause?

11 Comments

•

Sometimes the bug is hiding in the one place you least expect 😂 What's the weirdest bug you've ever faced?

2 votes
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@[m_sameer] this is an old one and wouldn't happen in modern times, but "back in the day" browsers didn't have developer tools, but there was a little scriptlet called "Firebug" that would let you inspect the page and do some of the tasks we now all take for granted.

I had a problem where there was a JavaScript error on the page, but whenever I used Firebug to try and catch it, the error didn't happen. It was like the observer effect for a web page.

Long story short, the error was caused by a console.log statement in the code and browsers didn't have a console, so it was undefined and threw an error. When I opened Firebug, it added a console and the log statement worked, so no error.

In the end, I created a "silent console" that would be applied to the page if no console existed, so I didn't have to wrap every use of console.log in a conditional statement.

3
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@[Steve Fenton] That's a fascinating example of how the debugging environment itself can change the behavior you're trying to diagnose. The console.log issue makes perfect sense in hindsight, but I can imagine how confusing it must have been at the time when opening Firebug actually made the error disappear.

I especially like the silent-console solution. Rather than working around every console.log individually, you addressed the underlying compatibility issue in one place. It's a great example of how a good debugging process can lead to a cleaner engineering solution.

Thanks for sharing the story, Steve. It's a great reminder that when a bug disappears during debugging, the debugging environment itself should be part of the investigation.

1
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Great topic. This is something almost every developer experiences at some point. Checking environment variables, dependency versions, and server configurations before deployment can save a lot of debugging time. Testing in a production-like environment is just as important as making the code work locally.

2 votes
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@[Next Big Creative] Absolutely. A production-like environment can reveal configuration and dependency issues that are easy to miss locally. I’ve found that checking those differences early can save a lot of unnecessary debugging later. Thanks for adding this perspective.

1
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Environment variables and identity are where I see this most in Azure work. Code runs locally with a developer identity and a local Key Vault setup, then fails in production on managed identity permissions or a missing app setting. Reproducing with the same container and the same IaC applied to a test slot catches most of it before it reaches production.

2 votes
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@[manojkagitha] That’s a really good point. Identity and environment differences can be especially difficult to spot because everything behaves normally during local development. Using the same container and IaC in a test slot is a strong way to expose those differences before production. Thanks for sharing the Azure perspective.

0
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This is a great list of different possible problems. Nice work.

3 votes
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@[Built2Win] Thanks, I appreciate it! There are definitely plenty of other production issues worth adding, but these are some of the ones I’ve encountered most often.

2
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Database permissions is the one I'd move to the top of your list. I had a feature pass every local check because my dev role could read a table the production role couldn't, and it surfaced as an empty result set instead of an error, so nothing alerted. Since then I diff the grants between environments before I diff the code.

1 vote
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@[nik-13] That’s a great example of how production issues can be misleading. An empty result set is especially tricky because the application can appear to be working normally while permissions are silently changing the behavior.

Diffing grants between environments before digging into the code is a really practical approach. It’s a good reminder that when production behaves differently, configuration and permissions can be just as important as the code itself.

0
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Muhammad Sameer
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