I was hoping

// setInterfaceValue sets an interface value to a concrete value,
// but first it checks that the assignment will succeed.
func setInterfaceValue(ivalue reflect.Value, value reflect.Value) {
    if !value.Type().AssignableTo(ivalue.Type()) {
        errorf("cannot assign value of type %s to %s", value.Type(), ivalue.Type())
    }
    ivalue.Set(value)
}

Could be changed to 
errorf("%s is not assignable to type %s", value.Type(), ivalue.Type(),
value.Type(), ivalue.Type())

I feel that the documentation on gob could be improved, though from previous discussions
it sounds like there is disagreement on adding examples for interfaces. While perhaps
independently this error seems sufficient, I found it confusing in the set of errors I
encountered while trying to understand how to correctly use the interface encoding, and
I did not fully realize what that meant until I looked at the source to see
reflect.AssignableTo was being used. I feel like changing the wording in this way makes
it more clear that "assign" is being used like in the language of the spec,
and not in a less precise manner.