What was the biggest mistake?

One of the most significant mistakes in programming is deploying code to production with a critical bug that leads to data loss or system downtime for an extended period. For example, in a Golang project, this could be a race condition in managing concurrent data access, which only manifests under high load and causes incorrect behavior.

Example: lack of proper synchronization when working with shared resources, leading to race conditions and unpredictable results.

An important lesson is to always use tools to detect race conditions (go run -race), write tests, and thoroughly review code before deployment.