package main
type I interface {
	foo()
}
func main() {
	var i I
	i.foo()
}

When, I run it, I get:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x104b332]

goroutine 1 [running]:
main.main()
	/Users/khr/gowork/tmp1.go:9 +0x22
exit status 2

Note the faulting address is 0x18. That's the offset of runtime.itab.fun[0]. The code is trying to load the function pointer from a nil itab pointer.
There's a subtle bug here - if an interface has lots of methods, that offset is potentially large enough to not fault when loading from the nil itab. We'll then jump to arbitrary code.

It's even potentially a security risk. I'm not too worried, though, as it requires an interface with >~ pagesize/ptrsize = 512 methods to even present that attack surface.

We do the right thing for methods that we go or defer, see CL 23820. I think we just need to do the nil check for everything. It should be easy to optimize out in the common case.

First reported on golang-nuts: https://groups.google.com/forum/#!topic/golang-nuts/MCBYxlaD-08

@cherrymui