The plane is a 1956 Skyhawk C172... Operators Manual states "Slips are prohibited in full flap approach because of a downward pitch encountered under certain combinations of airspeed and sideslip angle." This C172 has 4 positions on the flaps 10/20/30/40 degrees. For full flap landing, we use 30 (we don't go to 40). I had two notches of flaps in on final (20) and was high and decided to do a slip. Was a good landing but on short final (after the slip), my CFI looked down at the flaps as I pulled it to 30 degrees (once I was closer/over the trees) and said "you should not be doing slips with flaps". I responded that I read the POH and it stated not to do slips with full flaps (quoted above). He was curious and surprised so I sent a copy of the page from the POH to him, we'll follow back up later this week. What's the thoughts/consensus here? Thanks in advance. Fly safe!