A diode is a semiconductor version of a rectifier, but it does not isolate the circuit against any of the nasties that lurk out there in The Real World^TM. First off, any diode (or old-style germanium rectifier, for that matter) has a breakdown voltage, which is the point where the little bitty lectrons jump the PN junction chasm anyway, creating a closed circuit. Typically these 'reverse breakdown voltage' thresholds are on the order of 50-100v, and so either 110VAC or any level of static electricity is enough to pass a diode. The static charge generated by walking across a carpet can easily be more than 800v, although the current is rarely worth worrying about. A lightning strike nearby or any heavy rotating machinery like generators or motors can play havoc with the ground potential, though, so there are a number of very real potential trouble sources. In many of the third world countries, as in (still!) parts of the US, grounding is neither sufficient or complete, so extra care is highly recommended.