Not flossing your teeth can lead to cavities (half a mouthfull later....)
Styrofoam doesn't hold up very well in a microwave!
Eating too much can lead to a stomache – unfortunately I don't always seem to remember this lesson!
Chili powder is something that should be measured and only used conservatively (as opposed to just shaking some in like I just did – hooottt!)
When recipes suggest softening butter to room temperature, be sure not to nuke it until it melts since that consistency doesn't always work for every recipe.
Pouring fabric softener directly onto your laundry will stain it (oops!).
If you put away your clothes straight away instead of piling them up, they're less likely to get wrinkled. Same goes for why you should get clothes right out of the dryer.
Just because you're warm at the moment, doesn't mean you should leave your sweater at home. It will eventually cool down in the evening (most of the time), not to mention how heavily air-conditioned some places can be. Hence, tie your sweater around your waist or leave it in the car where you can grab it later if you want. This is also another reason why mom was right when she said you should always 'dress in layers.'
Caulk is difficult to remove from your fingers, but not as bad as super glue (hence, always wear gloves when glue is involved).
Always read the fine print (and some that's not even that small). That would have saved me accidentally buying two LPs instead of CDs, an oversized reel-to-reel player, and an earlier edition of a video game.
Make sure you know the difference between the lock and unlock position on the button on the inside of your car door (which could mean the difference between unlocking all the doors or locking your keys inside like I just did).
New addition to list:
Always check the weather forecast when visiting somewhere far from home. There's nothing worse than being woefully unprepared for temperatures either colder or warmer than what you expected. Of course it's not always possible to get an accurate forecast, in which case see 'dress in layers' above.