Maybe you could go back to your original needles for the final 5 inches of your blanket? That would at least imply that you followed a pattern of sorts?
Anyway, what baby is going to care? And I suspect the Mother will be simply delighted to get a hand-made baby blankey, no matter what!
So just learn from your goof. Likely it's not that much of a biggie unless you're a perfectionist knitter - in which case, I respect you and regret that you may need to frog.
Me, I'd make a good Shaker: I always make sure to include a small (?) mistake in everything I make - just to prove I'm not perfect!