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?