nil is not always nil

This is the most cited Go gotcha — and once you see the underlying model, it stops being surprising.

Interface values have two parts

An interface value in Go is a pair: (type, value). A nil interface has both fields nil. But a non-nil interface that holds a nil pointer has a type and a nil value — and that is not equal to nil.

GoSalin
var p *MyError = nil
var err error = p

fmt.Println(p == nil)   // true  — concrete pointer is nil
fmt.Println(err == nil) // false — interface has a type, even if value is nil

The most common place this bites is in error handling: a function returns a *MyError that is nil, assigned to an error interface, then compared with == nil. The comparison fails and the caller thinks there’s an error when there isn’t one. Return the interface type directly, not the concrete type.