You're working with someone (let's call them Kathy)that you've actually recommended to their current position. They're working with you on projects. The first few are just fine, things work out great. Then you hit one where there are apparent communication issues. You try to work through it all, and you do, but in the end you end up with a lot of work on your plate that really should have been taken care of by Kathy. But, you get through the work, and you don't really rat out Kathy. When asked, you say, yeah, things didn't go according to plan, but you hold back and don't really tell the *whole* story, put it down to circumstance and misunderstanding, and move on. Kathy, for her part recognizes that she sort of screwed things up, apologizes a lot, and tries to make amends. You figure that's all taken care of and that it won't happen again.
The next project that comes up, you find yourself in need of another person to work on it with you. You just need someone to do some basic work. We'll you're a project manager so who you ask is as your discretion typically. This time you decide to go ahead and choose Kathy, despite the problem the last time, becuase well, she's done so well before, maybe it's just a fluke. Your supervisor though is none too thrilled with your choice and actually takes some convincing. They've decided that they don't like Kathy, but you push to use her anyway. You get your way, the project starts up, and things seem to be going well.
About halfway through the project you get called into the office by the supervisor. The supervisor has had some discussions with Kathy over the previous week that have left them upset and concerned. The supervisor lets you know that since the project began, Kathy has been sending mail to the supervisor letting them know that the project that didn't go so well, well it didn't go so well becuase YOU were a poor communicator, weren't open to questions, and didn't explain yourself well. You also had unrealistic expectations and put off telling Kathy about major components until the last minute when she couldn't possibly complete them. Your supervisor now wants to have a long talk with you about the possibility of you lying about Kathy's performance, and what that means for your future with the company.
So... project gone wrong, then recomendation for further projects, then "oh, but this person was a dick manager" kind of stuff... all from a person who was originally recommended by YOU...
We had a long hypothetical "what if" sort of thing based off of this becuase someone I work with has had this exact thing happen and we were getting far too deep about it with the whole meaning of friendship and how trust is a bankrupt institution and so forth.