Problem
Attempting to attack a non-hostile monster, and refusing to do it when the game asks for confirmation, lowers the EP cost for whatever action you perform immediately afterwards.

How to reproduce it
1. Use the :t command to make the dynamic display show the energy cost of your last action.
2. Move a few tiles and note the energy cost displayed for doing so.
3. Find a peaceful monster somewhere.
4. Attempt to attack it a few times, and answer 'N'o when asked "Really attack the -foo-? [y/N]" every time.
5. Move to another tile once/attack an adjacent hostile/shoot a missile, and note the reduced energy cost displayed for doing so.
6. Repeat step 4, but refuse to attack the peaceful ~15 times.
7. Repeat step 5, and note the huge EP reduction.

Notes
I've only tested this by moving in regular dungeons, inside the underwater cave, attacking in melee and using missiles. Refusing to attack the non-hostile monster enough times makes the energy cost of all those actions drop to a minimum of 100. Just like the free healing bug, this can be very abusable when fighting a tough enemy - if you can manage to keep an adjacent peaceful monster from being killed (by e.g. putting a mook behind you and use Calm Monster, or taming dogs using bones), you can attack 10 times before your opponent gathers enough energy to hit once.