Leviticus 19:19 says: Ye shall keep my statutes. Thou shalt not let thy cattle gender with a diverse kind: thou shalt not sow thy field with mingled seed: neither shall a garment mingled of linen and woollen come upon thee.
Deuteronomy 22:11 says: Thou shalt not wear a garment of divers sorts, as of woollen and linen together.
What's so bad about mixed fibers? So "linsey woolsey" is obviously out; I presume fustian (linen warp/two cotton wefts) is a no-no as well. But what about poly/cotton blends?