What happens if someone cast Magic Jar (PH p.250) on a person who has been revived by a Revenance spell (Spell Compendium p.175)? Does it keep him alive after Revenance has ended? Does the caster of Magic Jar die in his place? Do they both live? Do they both die? Also, what happens if the person revived by Revenance casts Magic Jar?
The solution is all in the text of magic jar once we consider that revenance kills his target.
When revenance ends, provided the host body is still in range of the jar, the revived character dies and the caster's soul gets back into the jar. If the jar is out of range...
...which happens to be pretty self-explanatory.
If the caster is the target of the revenance spell himself, his soul gets into the jar and then two things might happen, depending on interpretation.
I personally favor the first one: when somebody dies, it's his soul departing from its body and going to the appropriate planes (depending on the setting).