If you weren't shifting to the large cog it can't be chain length (and it's quite easy to check for proper length) or limit screws, and a bad chain pin or master link is not only unlikely but quite easy to spot on the chain. You state you had just shifted, which sounds like the chain had seated on the cog properly. I don't see any way to apply blame to the shop, especially if your shifter is not in the large cog position. It's possible something flipped up from the road/path into the drivetrain.