Shield bug

That’s “stink bug” if you live in the US – I gather they produce a foul smell to deter predators, though I’ve never picked one up to find out. At first I thought this brown specimen was a forest bug, but the folks at British Bugs tell me that the green shieldbug sometimes changes to this colour in the autumn, presumably as camouflage amongst dead leaves. A good reason not to be too tidy in the garden!