Stating The Obvious...
When it says "12-18 Pounds" on a package of diapers, it relates to the size of the child, not how much shit the diaper holds.
"Yes" is the wrong answer to any questions asked by a wife/girlfriend/other significant other about whether the dress/pants/bathing suit she is wearing "make her ass look fat."
Saying "your ass makes your ass look fat" is also a poor response.
Leaving the television on so that you can watch the game/a Clint Eastwood movie while you have sex is just plain wrong. Even if she tells you that it is okay. It is, however, okay to use the DVR/Tivo during foreplay to ensure that no part of the show is missed. (Maybe.)
Having forklift jousts using cardboard tubes as lances is a really bad idea.
Having forklift jousts using pipes as lances is an even worse idea.
On a sailboat, when someone says "Jibe ho!" (or "Gybe ho!" on the other side of the pond), it helps to duck to avoid the boom. Especially when it is blowing like snot.
On a side note, reaching up to grab the boom, or fend off another sailboat underway in a race situation, is also a really terrible idea.
Using a hammer to separate scrap metal from the explosive part of an RPG is really bad idea. It did, however, get someone the 2006 Darwin Award.
Asking a police officer if he would mind checking for outstanding warrants is really stupid, especially if you do.
Saying "Whassup, my nigger?" is a really awful idea if you are Michael Richards (or otherwise white), and are visiting Roxbury, Massachusetts; Harlem, NYC, New York; or East Saint Louis, Illinois. Especially during Black History Month.
If you are white, and traveling on an airplane in America, you will be searched. Especially if the TSA screener has a choice of searching you, or a swarthy Arabic type (or a woman in a burqa). It is, generally speaking, a bad idea to complain vocally about this insanity.
If you take your family to Fenway Park, and sit in the bleachers, you should not be surprised to hear foul language, or to have someone throw up on you. If you have a Yankees shirt on, then expect the language to be even more foul.
Patriots fans are even worse.