Like the exec who brought his own wireless router and wireless dongle in so he could move his desk across the room where there was no network outlet. And thus compromised a high security network because he didn't know how to even do the basic securing of the network and it was now open to the world.

Or the General's aide (read executive / personal assistant) who entered all the phone numbers he regular rang into the memory of his mobile phone so he didn't have to carry around the book with them in them. When he lost the phone 27 unlisted phone numbers had to be changed and a few hundred people informed of the fact. The guy's personnel file was marked 'not suitable for promotion,' a real career killing move that one.

Then there was the guy who used his own notebook to do work stuff as it was more convenient and he wasn't authorised for a work issue one. Two years later he left the company but no one thought to check his notebook as no one knew he had been using it for work. The information came to light eighteen months later when confidential data on the notebook was revealed as still on the hard drive by the person who bought the second hand notebook from him and checked the drive out before cleaning it.

All these are pre BYOD days and BYOD policies, but they are the sorts of issues BYOD will see on the increase.