How can supplements be inadvertently contaminated?

In many cases the presence of a banned substance in a supplement product arises from inadvertent contamination. Such contamination can come from a number of sources eg.

1. cross-contamination between raw materials within the supply chain
2. impurities within the raw material (particularly herbals)
3. cross-contamination within the manufacturing process
4. contamination at the third-party (sub-contract) manufacture