Vetch seeds in sweet pea packet

More vetch, this time in the Royal Family mix from Premier Seeds Direct. I contacted them and they said that the contamination is due to the way they're grown (in a field, presumably sprawling along the ground), and the supplier has a tolerance of 2-3%. I understand it's hard to get the vetch seeds out since they look so similar to sweet pea seeds, but have you had any other varieties of flower or vegetable have random things pop up in them? The only one I can recall is a rogue cabbage in a packet of broccoli from Real Seeds.

Some time ago I had a packet of regular leaved tomato seeds from Premier Seeds and two of the resulting plants were potato leaved (this also happened with some other seeds but their name escapes me).  I told Premier Seeds but they didn't even bother to reply.

Not replying isn't very good form, hmm? I am curious if professional plant growers would be happy with 2-3% of their seed being duff. Would they have to overorder seed and then pay people to pick out thousands of vetch seedlings? My theory is that the vetch is only in the cheap mixes, and single varieties will be grown under conditions which will limit risk of contamination (hence the extra cost).