an effect that targets opponents placed on an enemy...

[Kiba]

if an enemy accidentally gets an effect on it that affects opponents only. is it the opponents of the monsters or enemies of the spell caster?

[MrCharisma]

What's the effect? (Eg: what's the spell you're using or whatever?)

Best guess it'd be opponents designated by the caster, but it probably will depend on exactly what you're talking about.