Huh. A couple months back, I helped someone replace a broken stud in their garage with some C-clamps and hammers. On a lot of vehicles, it's just "press the old one out, pull the new one in."

But there's a reason that r/justrolledintotheshop talks about "uggaduggas" as a torque spec - to make fun of the people who just let an impact driver pound away at things like lug nuts. Cross threaded? Screw it, more uggaduggas! It'll snap when someone takes it off? Not My Problem!

Oddly enough, it's not always righty tighty. I rode motorcycles that had axles that were the opposite (so normal rotation would tighten them), and it was well known on the forums that if you let a shop just go at it, they'd try to loosen it, tighten it, smash the spacer, and your wheel bearings would then fail in short order. I always either watched the shop take my wheels off, or just took the wheels off myself and took them in to get the rubber replaced (alarmingly frequently - I was riding a lot).

To be fair, it might not have been Costco's fault, it could have been the last guys to put the wheel on who impact gunned the fuck out of it, or shitty studs or lugs, or a combination of all three. That said, Costco should have had zero problems replacing a broken stud on the spot, any business who regularly removes wheel from cars should be prepared to replace a few studs here and there. It would be like hiring a guy to change your light fixtures but he can't finish because he didn't bring any spare wire nuts because he assumes their are already good wirenuts on the old light fixtures.

Well, the tires were purchased (and mounted) at Costco, and rotated at least once previously by Costco (since the path of least resistance for warranty maintenance is for them to have the records of doing the (free rotations) themselves; they'd have a hard time arguing in the event of a warranty claim that they'd rotated the tires incorrectly).

So history argues in favor of it being their fault. In fairness, they were apologetic and never tried to absolve themselves of responsibility.

Regarding "Costco should have had zero problems replacing a broken stud on the spot", I wish it were so (would have saved a bit of hassle), but which expert person at Costco is going to be capable of doing this? They seemed to have shop nearby that they farmed such work out to, but this problem occurred on a Sunday...

I would almost certainly have been comfortable driving it on the remaining lug nuts for a day or two until I could deal with it. The only way I can think of that I'd not just do that would be if it were a 4-stud wheel; anything with five or more studs shouldn't have a problem with one missing.

That said... I'd have wanted to pop into the shop and personally inspect the remaining studs to make sure they weren't on the brink of failure as well.

Costco sheared off the locking socket for my weird locking lug nuts, so I daresay they're more capable of some to just uggadugga it until it (usually) works.