There was one time many years ago where I was giving him his dinner shot, and I mixed up the vials of insulin. He ended up getting a whole lot of novolog, and very little levemir. I realized my mistake not too long after and started freaking out, thinking I was going to have to start pumping him full of carbs.
About 1 hour later his sugar was 300 and wouldn't come down. He ended up getting sick with a fever and needed the extra insulin anyway. That was the only time I made a mistake in dosing and we got really lucky!