I've heard it said that one can't be both omniscient and omnipotent; omniscience would preclude the ability to alter the future, after all. But I think it goes further than that; I don't think either omniscience or omnipotence are possible, even independently!

Can an omnipotent being make a rock so heavy that they can't lift it? If they can, then they are not omnipotent, because they can't lift the rock. If they can't, then they are also not omnipotent! It's an insoluble quandary.

As for omniscience, can an omniscient being's understanding contain its own infinite knowledge?