I was stacking my galaxy shots for a year before a guy at the star party pointed out my flats were wrong.
He asked if I was shooting them at the same focus as my lights, and I realized I'd been refocusing the scope first. Do you always match focus exactly, or is a close enough flat still okay?
What if that's the whole point though? Sometimes a flat, uniform look is exactly what you want for a specific mood. It's not always a mistake, it's a choice.