1. Being Indiscreet About Looking for a New Job: Meaning, don't search for jobs while you're on the clock. Anyone could walk by, see your screen for a split second and immediately know what you're up to.
Also, you should never APPLY for a job or send a resume from your work computer, because you're probably being monitored by I.T.
2. Gossiping: Plenty of people do it all day long and DON'T get fired. But, all it takes is one poorly timed comment while your boss is nearby.
Plus, if you gossip a lot, other people are probably gossiping about YOU. Any negative comments your boss hears, even if it's not true, could prevent you from getting a promotion down the road.
3. Taking Too Many Personal Phone Calls: Again, if you're on the clock , you're expected to be working, not planning your cousins baby shower or talking about March Madness to your old college buddy.
You might get away with it a few times, but once it's obvious that you're wasting a lot of time, you'll at least get pulled into your bosses office for a talk.