Some people just can't be trusted with secrets, for they simply can't control their mouths. Even if they have promised that they wouldn't tell anyone about the secrets, they eventually would.
I personally had one very unpleasant experience with one of these people.
About a few months ago, one of my friends suddenly asked me what my previous name was (I changed my name when highschool began). I didn't want to tell him— or anyone— about it, because I simply didn't like the name and didn't want people to associate me with that name. However, he kept pressing me about it, so eventually I told him, under the condition that he wouldn't tell anyone else.
But then a few weeks after, he suddenly started telling people what my previous name was. I gave him a warning glance, and simply denied what he said. However, he didn't take my cue, and kept telling people that he was sure what my previous name was. I was furious. If you could remember my previous name so clearly, why couldn't you remember your promise?
Fortunately, I kept my cool, and managed to make him look like a fool by letting many others think that he was lying (for he was known for his unaccountability). However, I still think that some people believed him, for why would one lie about such a thing?
I was very lucky that the subject of this incident was not important at all, for even if others knew about my previous name, it would just make me feel bad, and no real harm would be done. But sometimes these people could be dangerous. For example, once a girl casually told a teacher that another student was complaining about the teacher! I felt sorry for that student, for then his relationship with that teacher was probably ruined.
Some people simply can't keep their mouths shut when they should, and at any moment they can burst out some inappropriate line. Therefore, I should always be careful about what I say around these people.