Following up on something I said earlier: whenever someone says "Never do this", it either means "Never do this, unless things get really desperate/something unimaginably unlikely occurs". However, occasionally they really do mean "Never do this under any circumstances". The character they say this to will go ahead and do it anyway at some point, the difference being in the first case, it will be the right thing to do, in the second case, it will be the wrong thing to do.
When your main character has children, it must always be twins, first time off, first a boy, then a girl, setting the order of importance up right from the beginning. Nine times out of ten, the characters will all be surprised that the children are twins. If you must go with just one child, it will always be a boy.
And on a similar subject, whenever you introduce a female character as part of an organization, it can only be because that character will be of some importance later. Background characters are always male, why bother to make them distinctive, anyway?
And, on the subject of female characters always being damsels in distress: the female member of the party will always be the one to get grabbed and held hostage. After a few minutes of standoff, the female character will usually manage to get out of her attackers grasp, adding in a few choice blows for good measure. However, said female character will never manage to incapacitate a potential hostage taker before he (it's always a male) gets her in his clutches.
Anyone who goes through a door and slams it behind them without checking to see what's on the other side of the door is as good as dead.
Half the rulers in the world (three quarters in medeival times) regularly disguise themselves as commoners, and go out among them, usually just to check on the pulse of the community.
Any backup equipment (computers, power generators, tools, weapons, etc.) will always take so long to power up/bring in/switch on, that whatever crisis they could possibly be needed for would have resolved itself long since, one way or another. This incredible lack of speed it takes to get the backup equipment working will never be the result of actual sabotage, and in more than eighty percent of cases, it won't even be the result of poor maintenance or neglect.