Keep Your Mouth Shut When:

In the heat of anger

"He that is soon angry dealeth foolishly:
and a man of wicked devices is hated."

(Proverbs 14:17)

You don't have all the facts

"He that answereth a matter before he heareth it,
it is folly and shame unto him."

(Proverbs 18:13)

You haven't verified the story

"Seest thou a man that is hasty in his words?
there is more hope of a fool than of him."

(Proverbs 29:20)

Tempted to joke about sin

"Fools make a mock at sin:
but among the righteous there is favour."

(Proverbs 14:9)

Ashamed of your words later

"The words of a talebearer are as wounds,
and they go down into the innermost parts of the belly."

(Proverbs 18:8)

You want to appear wise

"Even a fool, when he holdeth his peace, is counted wise:
and he that shutteth his lips is esteemed a man of understanding."

(Proverbs 17:28)

It would destroy a friendship

"A froward man soweth strife:
and a whisperer separateth chief friends."

(Proverbs 16:28)

It would destroy a reputation

"An ungodly man diggeth up evil:
and in his lips there is as a burning fire."

(Proverbs 16:27)

When you may have to eat your words later

"Death and life are in the power of the tongue:
and they that love it shall eat the fruit thereof."

(Proverbs 18:21)

It's time to listen

"A wise son heareth his father's instruction:
but a scorner heareth not rebuke."

(Proverbs 13:1)

When you should be working instead

"In all labour there is profit:
but the talk of the lips tendeth only to penury."

(Proverbs 14:23)