Was it a short block?
If so, they install all your pans, covers and manifolds as it does not come from the factory like that. More of a chance for them to NOT get a good seal.
A full block (and more expensive) has everything but the starter and exhaust manifolds.
Still, no excuse but more an explanation as to the failure. Keeping it clean for a good seal is extremely hard to do.
I'm sure they'll get it right now!
HTH
~ Jeff