Are there any known instances of a lecture recording being used against an instructor unfairly?

I doubt doing things that could be construed as "out of line, incorrect, inappropriate, whatever" really improve your lectures in a significant and sustainable way. In any case, there are better ways to be interesting and entertaining [1].

I think everyone understand that instructors aren't concerned about copyrights. They're concerned about having to improve the quality of their lectures, or have a permanent record of lectures they're not really proud of.

[1] Richard Buckland, UNSW: http://www.youtube.com/watch?v=2bLCjMA0YlE