Weirdness in B-flat

A few weeks ago, I had my piano tuned. It was waaay overdue. The tuner seemed to do a very thorough job, but within minutes after he left I noticed something odd about the tuning. Everything sounded just right...except the B-flat major chord. There was something gratingly wrong about B-flat. It didn't matter what part of the keyboard I played it in: high, middle, low, it had the same weirdness to it that actually buzzed in my ears.

I decided to let it go for a little while, to see whether the instrument would adjust by itself. But today I finally gave in and asked the tuner to come back. I don't know if it's my imagination, or if my hearing is shot after playing an out-of-tune piano for too long, or if it's just another case of the Princess and the Pea; or, perhaps, my tuner uses a tuning system that can sound just right in every key but one. Maybe he overlooked B-flat when he was checking his work, a fateful omission. Or, maybe something is wrong with the universe! (Thank you, Dr. Crusher!)

EDIT: This is an obscure reference to Star Trek: The Next Generation, episode "Remember Me," in which Dr. Crusher made the impressive observation: "If there's nothing wrong with me, there must be something wrong with the universe!"