A team of four developers spent four hours debugging a production API issue caused by misunderstanding Go slices, which led to duplicate items and unexpected data mutations. The root cause was improper handling of slice copying, appending, and shared underlying arrays. The article shares practical patterns and fixes used in production to avoid these bugs, such as always returning new slices after append, explicitly copying slices when independence is needed, and careful capacity planning to avoid reallocations.

Jun 8, 2026, 11:00 PM