Yep, had one at a local gun shop just the other day. Although I'm 52, I was the youngest guy in the shop by 2 decades (only exagerating slightly), so when one of the old-timers noted that I was examining several boxes of bullets (pretty much everything I could find in .45 caliber), he "offered" the benefit of his expertise.
My first time in this shop, I'm not out to offend anyone; I listened to him prattle on for 15 minutes or so.
Now let me be the first to admit--I only reload for .45acp, and am a newbie at that, so I don't know a lot. In fact, while looking for a .45 Colt revolver, I bought a set of dies for .45 Colt and bought 100 bullets in .454 dia.
But when he tried to tell me that .452 bullets were for .45acp and
.458 bullets were for .45 Colt, I bit my tongue. When he was "done" (loosely translated as when the door swung open and he diverted his attention from me for a second or two and lost his head of steam) I thanked him for his time, wished him a Merry Christmas, and put three of the four boxes back on the shelf (purchasing the .452 255 gr SWCs).
What blew me away was that the shop owner stood there, listening to most of this, and never said a word. If I were ill-informed enough to buy, try and load 300 gr .458 projectiles into .45 LC cartridges, and somehow succeeded, I can't imagine they would fit into the cylinder...which would be a good thing, because if they did, I'd hate to think what would happen if I could touch one off.
At this point (three days after the fact), I'm thinking I should have said something. If this is any indication, this guy's advice isn't just bad, it's potentially dangerous.