Normally, I wouldn't care what sort of games you buy. I've seen  folks buy some of the worst, most awful videogames I've ever seen (like the now thankfully illegal 'The Guy Game' ) but I didn't say or do anything because I'm the sort of person who figures people have their own reasons to buy and play whatever games they want and none of it's really my business.
 
Then there are times when I just can't stop myself from intervening. One of those times was two years ago (2008), when this brother-sister duo (one looked 12 while the other looked about 15) wanted to buy an MMORPG that was "like World of Warcraft but not 'cause mom said we couldn't play it anymore". Trouble was, the game they picked up wasn't an MMORPG at all: It was Medieval 2 Total War, a strategy game.  Standing right behind them in the line at the cashier, I just had to say something and tell them they were picking up the wrong sort of game.
 
As it turned out, they took it because "it looked just like WoW. Look at all the swords and armor!" and thought it was an MMORPG because they saw some screenshots of it and "saw lots of dudes PVPing in a huge map!" I could've exploded with a barrage of insults that basically could be summarized "HOLY CRAP YOU KIDS ARE DUMB" but I decided not to and instead asked them why their mom banned them from playing WoW in the first place. There was a very good reason: The kids weren't doing well in school and their mom noticed them playing for at least ten hours a day, not doing their homework and such. They promised their mom that they'd stop playing WoW, and their mom would let them play a different game but only for an hour a day at most.
 
 After a short conversation, I convinced them to find a different game. I'm not an MMO fan, nor am I knowledgeable about the subject, so I couldn't recommend them what to buy instead. One of the store clerks helped them out, and I myself proceeded to purchase a copy of Battlefield: Bad Company.
 
There were a couple of other times when I intervened. This one was last year: A nine or possibly ten year old boy had convinced her mom to buy Scarface: The World is Yours for his PS2. The clerk at the desk asked her if she was aware of the game's 'Mature' rating. She said she didn't know that it wasn't meant for kids and then asked her son if there was swearing in it. Apparently, all the violence in the game wasn't a problem with her but swearing, for some reason, was.  The kid just said "Nope, not a single bad word!" 
 
At that point, I just had to tell her: "Lady, I'm sorry but your son is lying to you. That game's got a ton of swearing." The clerk joined in and backed me up, telling her if she didn't want  swearing then she should not buy it. The kid was understandably upset, screaming at us "WHY ARE YOU RUINING MY LIFE?!" or something like that. The mom just grabbed his arm and yelled at him as they both exited the store.
 
If the mom really knew what was going to be in the game, and was okay with everything, I would've just shut up.  If the parent's okay with all of it, what the hell, sure, go ahead. The kid was just lying and sooner or later his mom would've found out about it anyway.
 
The clerk, Ian (I think his name was Ian, I can't really remember) thanked me for saving him from another angry parent complaining about buying a game they shouldn't have. I immediately asked him for a discount, but sadly he couldn't. "Wish I could, but I can't. Store policy. Besides, you already got a discount card."
 
I shrugged, and eventually left the store because I couldn't find a copy of Disgaea 2 there.

I now ask you: Have you ever intervened? Did you ever find someone about to make a mistake when it came to buying a game and then do something to prevent it?

I've heard riddicolous stuff like "IS HALO OUT FOR THE WII? BUT I HEARD IT WAS" being asked to the store clerk and every time I've just facepalmed and left for another store. There has never been a situation where I've really had a chance to actually intervene. I've seen people my age who had no clue what the hell they were talking about, but the clerk always saved their asses (A gamestop that doesn't milk cash? You read it here first). 
 
 Maybe I don't pay attention to people around me enough but man, I can hardly afford enough games to support my interest while those brats walk in knowing nothing but having enough cash to wipe the shelves in the store. If they want to waste their money by not making research, then that's their problem.  
 
I don't want to come off as a dick here, but people who are my age who still apparently don't know the difference between "My horse and me" and Halo or Mass Effect and just throw money at gamestop until the stars align and they pick out a good game at random just tick me off. I understand not everyone is interested in video games like me, but if they're putting out that much money on something they should be able to at least google it before they do some stupid shit. 
 
I've also heard "where can I buy a controller for the Playstation 3D" and when the clerk handed over a Dualshock 3 the guy was like "No this isn't a 3D controller man gimme a 3D controller" I mean, wtf?

Not really, as I would't want someone doing it to me. And besides, we learn from our mistakes.

never happened to me. I tend to only hang out in game stores for 10 minutes tops and usually don't buy a game. the only thing worthy of mention was during the midnight launch of MW2 where GAME had a promotion for trading in a game from a list to buy MW2 only half price and seeing a guy trade in Uncharted 2 which is one of my favorite games of the generation. But yeah apart from that there's nada

You're a douche, that's all I can gather from that remarkably long and shockingly written wall of text.

I can't remember stopping someone from buying a game, but I can recall many times when I convinced friends to buy certain ones. 
 
And, by the way, isn't Scarface the game with a button exclusively  for swearing? "Press X to say 'Fuck'."

I once was trying to buy a game and a guy came up to the clerk beside me and asks if they had Call of Duty World at Warfare 2. The guy serving him asks if he meant Modern Warfare 2 and he said that he already had that and wanted World at Warfare 2. The guy serving me by this stage started to laugh and could not contain himself as he tried to ask me my address as I had forgotten my loyalty card. The guy then walked off in a bit of a grumpy mood as the clerks were "useless" and then proceeded to approach my friend and tell him how great the new Fifa game was and would he know where to find World at Warfare 2. 

Stupid people cannot be avoided, protected or killed.  An attempt to do any of those things will only lead to frustration.

I actually used to work at a Babbages before they went out of business and I prided myself in intervening in purchase decisions.  Parents in particular are very easily manipulated by game boxes or similar sounding titles into purchasing the wrong things for their kids.  Sadly, being a knowledgeable game store clerk that gets the customer what they want (or Best Buy, where I also endured a stint) is pretty much mutually exclusive from the store making money since guides, pre-orders, and service plans are what makes money.  So we're left with GameStop.  Sigh.
 
Lately I don't do much in-store shopping, though I'm generally the go-to person for my non-gaming friends on purchase decisions.

I wouldn't give my opinion in any of these situations unless it was asked for.

I have wanted to at times, but I haven't really. And these days I don't really get the opportunity as I usually just walk into a store, get what I want, pay and leave. No time to hang around, I've got dudes to shoot in the face.

I've been asked about a game once or twice while in a department store. I dunno if I just have a face that makes people talk to me or not.