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The Go bugs I keep hitting come from holding something different from what I think I'm holding. A slice is a window, not data. A map doesn't guarantee addresses. A string isn't a character array. This post opens up the layout of all three.
You return a nil pointer and err != nil comes out true, because an interface value has two slots: a type and the data. From that two-slot layout to method sets, where interfaces should live, and the cost of dynamic dispatch.
Following how compiled, statically typed code turns into runtime wins, using Go: memory layout, compiler optimizations, escape analysis, the GC, and the scheduler, each checked with tools.