Don't ask me how this happened, but I drilled the holes in my Engine Mount for my Sig Kadet Senior ARF, and the holes are 3/8 forward from where they should be. Engine mounts great, but it 3/8" futher forward than the instructions say it should be. Is this going to be a problem?

It might cause a slightly forward CG, but other than that I don't think it's anything to worry about. You should be able to shift the battery around to compensate or you can always drill new holes.

Why don't you plug the wrong holes with set screws, loc tite these in, and re-drill in the right location?
Still, if the holes are 3/8 mis-drilled, a slight forward CG will not hurt anything at all.
Good Luck and Happy Landings

Don't sweat it. Yes, it changes the moments on the aircraft and you may have to move the battery pack rearward slightly to compensate or add a half-ounce of lead to the tail, but the airplane will still fly great.

The sig seniors are tail heavy so I would not worry about it. You will probably be moving the battery up front anyway.