My machine with Lion installed failed to wake from sleep the other day, and all sorts of nastiness was in evidence after a power-button shutdown and restart.
Not to worry, I thought, I'll use Time Machine (TM) to restore my Library folder to its pre-problem state and perhaps that will solve the problems. This seemed like a good idea, but TM refused to cooperate, informing me that the operation could not be completed because I did not have sufficient permissions to change my Library folder.
How could this be, since I operate with an Administrator account?
(As to restoring the machine to normality, it took two restarts into Safe mode to sort things out, at least to an acceptable extent — I've had to re-enter license details in several apps when they re-launched afterwards, and almost all my preferences had reverted to their default state.)