the chain? that's some harsh carelessness on their part. maybe not carelessness... just inattentativeness. that's bad.
from my experience working in a bike shop as well as being a bike customer, a good way to judge a bike shop is not whether or not they make small mistakes (i mean, they shouldn't happen, but they do), but how they correct them. if they're defensive or if you get a bad vibe (which can come from a grievous customer <i>giving</i> a bad vibe, too..), then that's good solid reason for being pretty disappointed. but if the guy smacks his head and says, "wow, that was a stupid mistake, i'm really sorry. here's a chain, i'll put it on while you wait, please come again, sorry it happened"--then i'd get a better sense that they're a decent shop, regardless of the mistake. cause they're honest and real about fixing their mistake.
once i did a freakin' stupid thing, i wrenched on a kickstand over the derailleur cable (thought it was out of the way. it wasn't. i didn't double check). the customer, who was buying a hybrid for her brother, was pissed the next day, i fixed the mistake and threw in a waterbottle cage, and she wound up coming in the next week to buy a bike for herself, and was really cool, too. so.