MSD

At least take the time to do a quick Google search of '[product name] reviews' before you buy something. Don't let confirmation bias sneak in and convince you to believe the one guy who gave it five stars when 65 gave it one, he's either a troll or someone the manufacturer sent out to give their product good reviews. Multiple identical reviews are marketing people or bots.

College students: If you don't think your dorm room smells funny, you are the smell.

Infomercial and As Seen on TV product sales people will not pick up a product and sell it unless they feel they can sell a minimum of 10,000 units, and the typical markup is over 500%. You can get something just as good as most of them for a lot cheaper, and the ones that you can't are probably so crappy that nobody bothered to copy them. Billy Mays would never sell you a product he wouldn't buy himself, but you can get cheap versions of stuff like Oxy Clean at any discount store.

The Sham Wow is just a chamois. You can get the same thing at Wal Mart for $7 because you're not paying for Vince's meth addiction. Yes, it does work, the reason yours didn't is because you didn't wet it and wring it out first. Also, don't put it in the dryer.

Hydrogen peroxide gets wet or dry blood stains out. Test it on an inconspicuous area first. No, under the couch is not an inconspicuous area, your wife will eventually rearrange the furniture.

When you get new carpeting installed, ask the installer for a few remnant scraps so you can have an inconspicuous area to test cleaners on when someone spills something.

To clean your microwave, put half a cup of baking soda in a cup of water, microwave it until it boils over, and sue the hot gritty residue to scrub off all the hardened on food. Then use the rest to clean laminate countertops. Do not use it to clean stone countertops.

When baking soda just doesn't take care of the stench coming from your shoes, fill a spray bottle with rubbing alcohol and soak the insides. Let them air dry and the bacteria will all be dead.

Your feet will smell a lot better if you use the hairdryer on them for a minute after you towel dry them.

If you are out of college, don't hang posters on your walls unless they're autographed originals or you worked on the movie and have them in your office. In that case, they should be in a tasteful frame.
The fruit fly trap I use is vinegar with a paper cone taped in so they can go down the cone but have a hard time finding the way out. Apple cider vinegar worked well for me, we had an infestation, and I was literally pouring out a sludge of dead fruit flies every morning.
Hot water freezes faster in some circumstances, but the likely reason in most household freezers is that the low temperature and humidity lowers the vapor pressure of water enough that a significant amount evaporates.
This is mostly a myth, but Nalgene bottles have been BPA-free for a while now, and combining one of those with a filter filtered pitcher is a lot more environmentally friendly and cheaper.

Speaking of Brita pitchers, put it under the faucet and set it to a trickle while you do something else. Come back in 5 minutes and it will be full instead of filling the top, letting it fill halfway, refilling the top, waiting again, and refilling the top once more before putting it in the fridge.
Don't touch your face or genitals for at least an hour after cutting peppers. Don't touch anyone else's either.
More general than that, just don't mix any household cleaners. Even if it's safe, if they didn't work, people wouldn't buy them, and if a combination product worked better, their teams of industrial chemists and professional testers would have figured it out and sold it to you in a new and improved form that costs more.