Actually, I'm hard pressed to remember stupid advice from LFS employees/owners. I can remember my own embarrassing moments though:
In 1990's: Arggh, no, no. no, you have to bag the Tridacna clam under water bc exposure to air will kill it. (Met with polite disdain, but no argument, from clerk at New World reef store in Manhattan.)
In early 1980's: I've got an algae problem in my 55. Will that 9" Naso Tang handle it? Ended up with algae blenny thanks to clerk in LFS in Sheepshead Bay, Brooklyn. Blenny did not make a dent in my algae but did not wipe out tank either.
My personal worst in late 1980s. In my defense I was working 18 hours a day, seven days a week, as a robo-lawyer on Wall Street and had read an artlcle in FAMA re Lee Chen Eng and his natural tanks. So being an arrogant a**hole, I wanted to instantaneously transform my FO SW tank to a reef tank. (Wait, what kind of defense is that?)
So getting home comparatively early one Saturday night, I went straight to LFS that was selling live rock, and announced that I wanted to convert my 55 gallon FO tank to a reef tank so give me 50# of live rock. Despite dire warnings from clerk and owner in Flatbush Ave. store, I bought the rock, and that night, rolled up my sleeves, removed the old lava rocks, and replaced them with 50# of stinking live rock. And within days, everything in tank was dead.
Jeez, I was a tool. Is it possible that there are as many toolish customers as employees in your average LFS?
In fact, it seems like when I visit local reef stores in tri-state area recently -- I don't shop at chains-- I encounter more often than not knowledgeable guys who are eager to engage in conversation/debate about what coral/fish/etc. would best suit my tank or why they would not. And I usually lose any debate.
So there are folks who work at local reef stores who contribute their knowledge and experience to help customers make the right choices. There may be some tools among them, but IME the non-tools are far more common.
Mike