I had a CF seatpost in a CF frame that kept slipping. So I kept tightening the clamp down some more. And more. Finally the split in the seatpost clamp was metal to metal and the seatpost was crushed. Did I mention that it wasn't a normal round seatpost? No it was a much more expensive teardrop-shaped seatpost.

Lessons learned:
1. Bought a little torque wrench that would go down to the recommend 5 nm setting.
2. Tacx Dynamic Carbon Bicycle Assembly Compound - which is what I should have used in the first place.