The rain spell isn't having the effect it should have.
Casting it when those small bomb guys are around doesn't put their fuses out.
Casting it in cold weather doesn't freeze everyone in the AoE.
Casting it when a character is on fire doesn't put the fire out.
Fires that should be going out when rain is cast doesn't always go out, like torches, campfires and fires on the ground can linger for as long as the rain spell lasts.