From a DM perspective, any ability that is not intuitive and does not flow naturally when running the monster. This is obviously quite subjective and will differ from DM to DM, but some personal examples are:
1. Regeneration for lycanthropes in 4E. I keep forgetting to apply it since I come from a background of damage reduction (in 3E) or not being able to hurt the lycanthrope at all (in 2E and earlier editions) without using the right kind of weapon.
2. Recharge abilities in general. I keep forgetting to roll for the recharge since I come from a background of monsters having either at-will abilities, or abilities that are limited by a fixed number of uses per day.
3. Certain triggered abilities. Again, if the flavor attached to the interrupt abilities is not strong enough to fix it firmly in my mind, I often find myself forgetting to use them when they are supposed to be triggered.