Herbal doesn't equal harmless. A tablespoonful of clove oil is enough to do real damage to your kidneys, and two will (according to my NHS doc) most likely kill you. Rosemary oil is toxic in large doses, so is nutmeg oil. Virtually all oils from 'woody' herbs (eg. rosemary, thyme, lavender) will give you a horrible stomach upset if you take too much, and I suppose they could kill you if you didn't realize and get treatment for dehydration.
Finally, willow extract (usually in the form of salic acid, I think) is the active component of asperin, and as such, can kill you just as overdosing on painkillers do.
Non-lethal, but not pleasant, are St John's wort (treats depression, makes the pill stop working), meadowsweet (good for stomach ache, but also makes you miscarry) and overdoses of evening primrose oil (er .. it's a laxative, I needn't say why taking 60 might make you wish you'd not bothered).
Lavender, rosemary and evening primrose oil are common components of herbal sleeping pills. Some also contain various willow-derived compounds, and some (particularly the ones that are meant to be pain-relief as well) contain clove oil.