Borer - What is a Mixture?

The following question was sent by one of the Chabura to Rav Ostroff:

What is the definition of a taroves [mixture]? Does borer apply to all items? In Siman 319:3 the Mechaber writes "two foods which are mixed" and the Mishna Berura (15) adds mixtures of different types of keilim or clothing.

Rav Ostroff replied:

Your question is the hardest issue of borer!! Rav Neuwirth gave a shiur and mentioned 12 cases of where he cannot define whether items are in a mixture or not. One must determine whether jumbled items are together and a mixture, or separate; it is hard to determine.