"I don't know" is an answer we should hear more often
We've all heard the adage "It's better to remain silent and be thought of as dumb than to speak and remove all doubt." While this is certainly good advice, I don't think it's entirely correct. Let me digress for a moment...
Tonight, I was doing some last minute Christmas shopping which was the perfect excuse to pick up another computer to use as a Windows Home Server. The sales people were particularly aggressive and every time I just stood there looking at stuff for more than a minute, someone would come by and ask "Can I help you find something?" Each time I said "I'm just looking" , they'd press me. It was honestly like they had a sales quota or contest going on. On three different instances, I had someone try to explain something to me in such clueless imbecilic terms that I doubt anyone would believe if I told them about it. I was 1/2 tempted to turn on my phone's voice recorder and go ask for clarification on a few things I had just been 'taught' just so I'd have proof that I wasn't making up what I heard. And in large part, it seemed to be inspired by one guy. He was the ideally stereotypical computer guy at a big box store. He would walk around and butt into conversations people were having and would then enlighten people with unsolicited advice. This in turn seemed to make the other guys feel compelled to not look stupid, which caused them to play buzzword bingo and talk about things they honestly had absolutely no clue about.
I could only think to myself "Some day, these guys are going to reflect back on their time here and when they realize how much BS they pulled out of their butts, they are going to cringe with shame". In each case, if they had simply answered "I don't know", I would never for a second have thought they were idiots. In each case, I had spent a few hours reading up on the stuff I was looking for and even then I had only a cursory knowledge of the stuff. So if some random guy at a big box retailer didn't have a clue, it wouldn't have surprised me in the least. Instead, I started to think "What a f_______ tool" Not just b/c they were kind of invading on my shopping experience, but b/c they talked with such an air of authority. And I started to think, 99% of the people that come in here are just looking for a good deal on X. They know they want a computer or router or flat screen and want to get one and not get ripped off. Instead, they get these longwinded lectures that invariably makes them feel a little small and in many cases, the guy doing the pontificating is just freestyling. And I started thinking "If you're going to force me to accept your help, can you please just stop insulting my intelligence quite so badly. I mean, being told that Windows Home Server is actually just Windows Vista Ultimate with a few Registry settings changed was bad enough. But when I asked if they had any Windows Media Center extenders and was told "there isn't really any product called a Media Center Extender per se, it's just a generic term used to describe routers that can stream media and then having him point to the router rack and say "All of these are essentially Media Center extenders" - that was too much.
I'll say this, unless you answer "I don't know" to something that you should unquestionably know as part of your job, it's really doubtful anyone will walk away and think you're a moron. Look around the internets and see what people often say about the Geek Squad or FireDogs or whatever. I only found a few examples where people complained that the sales person said "I don't know". But there are zillions of stories about some know-it-all feeding them a ridiculous line of BS. And the point is always "The guy was such an idiot he didn't even realize how utterly stupid what he was telling me was". A few people might be fooled by this routine, but do it enough and you're going to look like a fool in front of most people, and you're not just going to look a little foolish. You're going to look foolish and be thought of as a BS artist - a rep that's really hard to shake once you get pegged with it. In most cases, there's no shame in saying "I don't know" and there's always less shame than trying to play it off when you don't have a clue. If I ran one of those big box stores, honestly, I'd make new employees say "I don't know" 200 times just so they realized they could say it and the world wouldn't come crumbling down. And if done in each case that someone didn't know something, I bet very few people would walk out of my store talking about what idiots I had working for me. (Ok, I wouldn't actually make them say it 200 times, but you get the point).