Teaching Skills prevent monsters from Learning skills
I taught my Levi Tsunami at level 22 to help with TM Barren and decided to get rid of Erupting Spring in slot 2 (big mistake). I also taught him rain prayer and soak in slots 3 and 4 (not relevant). Slot 2 is where Levi learns Water Torrent at level 50. So when my Levi got to level 50 it did not learn Water Torrent. My plan was to overwrite infliction that was still in slot 1 but it didn't give me to option.
Take away: If you teach your monster a skill with a skill book in a slot where they are going to learn another skill, they won't learn that skill.