Enantiodromia means the tendency of things to turn into their opposite. It is one of my favorite words because it is so useful. For example:

A government established to protect freedom will tend to become a government that rules by slavery.
An anti-drug policy will tend to increase the use of drugs.

Both of these are examples of enantiodromia in action.

Otiose means "serving no practical purpose". I ran into this word in a computer science article where the author casually dropped it into the flow, causing me to go "iuh?!"

Etymology:
Otiosus is Latin for idle.
Otium is Latin for leisure.