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They've got fairly aggressive automated systems for identifying and flagging material that's registered with rights-management agencies. It works for both individual tracks and material featured in mixes. I've even had my own music taken down preemptively before since the label had already registered the forthcoming track. I was able to fill out a form with some personal info and they put it back up pretty quickly, but it was still annoying (though I completely understand why they have it set up that way).

Not sure how other people are confounding the detection algorithms. They're even resilient to changes in pitch. As a result, a lot of DJs I know can no longer post mixes to soundcloud and have to use other means.