That is the type of mistake that could easily happen at any gun shop. Even very knowledgeable sales person could easily see 357 on a box of ammo and pick up the wrong one by mistake. I've seen similar things happen with people buying 375 H&H mag when they need 375 Winchester ammo. With 5-6 different 300 magnums it is a common issue with someone knowing the difference, but still picking up the wrong box.
That opposite but exact same thing happened to a hunting buddy of mine and me at a well known, popular gun store in Los Angeles, years ago. My buddy asked for .375 H&H Magnum and the clerk handed him a box of .375 Winchester. My buddy had to explain the difference. Finally the clerk found the proper ammo so my buddy had him open the .375 Win. ammo box and compare the two cartridges. The clerk was embarrassed .... but enlightened.
Stuff happens even at the best of gun stores.
L.W.