Not potion IDsAs cool as Reika's mods are (except Geostrata) They are very temperamental when you fuck around with their IDs, I think Reika takes his no hand holding mentality to every aspect of his mods including ID conflict resolution.
That being said it does give you a pretty clear error message as to what the problem is, but there are other mods that will generate a sort of ID conflict report and even suggest free ID ranges.
Luckily this is a thing of the past with post 1.7 minecraft.