It's human nature to want to speak your mind or share news and gossip, but there are some pretty compelling reasons why you should refrain from talking in certain situations - especially at work. These are in no particular order.
When you want to gossip.
Saying something negative about someone (even if they can't hear it) is never a good idea. It can make you seem petty and mean.
When you're mad.
If you're upset, take a deep breath and wait it out. You may say something you regret and you aren't thinking clearly when you're upset.
When you want to correct someone.
This is especially true with regards to a superior or people new to your group or organization. Be the bigger person and avoid embarrassing someone, unless it's a grievous error that could cause financial ruin or have a dangerous outcome.
When you've set a big goal.
Some people think that having other's know their plans means they'll be more likely to see them through but research shows done in 4 different studies showed that people who kept goals to themselves were more likely to achieve them.
When you're drunk.
This is a tough one, because you tend to lose your filter after a few but try to stop yourself from be inappropriate or brutally honest. It's also recommended that you never have more than 2 drinks at a work function.
When you're about to complain.
No matter how tired, sore, or overwhelmed you are, keep it to yourself. No one likes a whiner.
When someone is being a jerk.
Everyone has bad days and difficult situations at some point in their life. There are people who don't handle things well but instead of pointing out how stupid or irritating they are, try being kind - you'll feel better and it might just defuse the bad behavior. Saying something negative to that person will only make it worse.
When you want to fill the silence.
Silence might be awkward but sometimes people need time to formulate what they want to say or how they want to respond - jumping in might hinder that process. You might also end up saying something dumb or irrelevant and be perceived as being self-centered.
When the deal's not done.
Remember that old adage "Don't count your chickens before they hatch"? Wait to share details about a deal until everything is signed, sealed and delivered. If it falls through you'll feel lame.