Yarrow Wildcrafting: Good and Bad Varieties?

I was about to go yarrow collecting but was asked if I knew the differance between the good and bad yarrow. I was told that some yarrow can be poisonous. I had never heard this before and was wondering if you might be able to shine some light on the subject. I know that there is yarrow with yellow flowers and yarrow with white flowers but that's all.

It's news to us too. There are no wild Achillea plants we know of that present any danger. Probably the advice stems from the fact that members of the carrot or parsley (Apiaceae or Umbellifereae) family bear a superficial resemblance to yarrow flowers, sharing the flat flower heads, white colour and size. Some wild species of the carrot family are indeed poisonous, including the notorious poison hemlock and others.
It is essential to know your botany when you collect from the wild. There are countless risks in misidentifying plants, particularly at times when the flowers – the most characteristic part taxonomically – may not be present to aid in making a positive identification. Even professionals can easily make a mistake when the flowers are absent.
A good identification manual for the area you are working is also essential. Beware of books that claim to be an identification manual for the herbs in your area but omit all the non-herbs. It is the what is not considered a herb that you need to know about just as much as the herbs.