It sounds like I'm being very petty bringing these kinds of thing up, but I'll go into a bit more detail. This particular employee was extremely critical of fellow employees at his own level, and would make extremely overt attempts to ingratiate himself with senior employees and managers. We used to interview potential candidates for junior positions together. One very odd behaviour that he'd exhibit that really rubbed me the wrong way was that when we gave the applicant whiteboard tests, he would make such a concerted effort to demonstrate his superiority to the applicant by painstakingly solving and explaining the problem in gory, patronising detail before their eyes. That was how I saw it anyway, I felt that he would relish in any minor victory over other employees. I guess they call this an 'inferiority complex'. In retrospect, I feel that I should have known that someone displaying traits like these wouldn't miss the opportunity to drag me down to build himself up.

The world is full of fairly nice people who will spill anything you said to them to anyone for any reason in the workplace. And I'm not cynical about people, I just think it's the truth.

There are no warning signs. Just pretend everything you say to a coworker is on a public ledger.

In some cases the true, actual friends you make in the workplace, the ones you'd still hang out with years after your employment, ca be trusted. But it's still probably better to not tell them anything that could make you look seriously bad either.