When your fighting a Monster and you pick something up from under it, such as insides, the enemy dissapears. If you take a steo anywhere it will reapear. This also happens if soembody else grabs an item from under a monster and you see it happen.
This is a well known bug, easily avoided, and has been addressed this same way many times. Don't worry about it they know. Oh and bugs are to be reported to bugs@illarion.org
Most graphical or "inability" bugs are cured by simply cntrl+dragging your character; a simple solution for those who are awaiting the fixing of the bug in the future.