Here are several examples. Mind you, whether you believe it or not, a lot of the people on this forum know each other irl and also know me. Some of them were my friends, so some of them will recognize themselves here in this post. I won't use real names, to protect the guilty.
One of my girlfriends wanted to commit suicide. She told me I couldn't tell anyone that she was feeling that way. Fine. One of my beliefs at the time was when a really good and cherished friend asks you to keep something secret, you do. So, I did. Time went on and I did everything, I mean everything in my power to help hoping she might get better, but there came a point where she actually started planning how to do it. Of course, she told me these things and I then became a nervous wreck. Finally, I made an appointment with the psychiatrist where she was getting therapy (no, she didn't tell her therapist she was feeling this way, only me) and I told the psychiatrist what she was feeling and planning. Then I got out of the relationship. So, I guess I was wrong to try to help her, and according to her in a bitter letter she wrote to me months later, I was also wrong to have broken the bond of friendship that we had. Until her suicidal tendencies started showing up, we were the best of friends, I really loved her. So, you see, I tried to help. But it became too much for to bear. DOOR SLAM.
I had a male friend who was my best friend. We talked everyday on the phone and would also meet once in a while. One day he said he was going to California and I never heard from him again. Later, I found out he was a con-man. I found out that he was an actor and was not only the person I thought he was, but also many other of the friends I had made as well. He used makeup and prosthetics to pretend he was someone else, and let me tell you, when someone is a Holly wood actor, there is no limit to the access they have to makeup and prosthetics they want to either purchase for themselves or borrow for a given role they want to play, irl. IOW, he was at least 3 other people, unrecognizable to me, and so I was telling him about things that he already knew about, since he WAS the person I was telling him about. If you don't think this is possible, think again. DOOR SLAM.
One of my other friends was his twin sister. I didn't know they were even related, until I found out years later. She befriended me and also used the prosthetics and makeup trick. So, I knew her as one person and then again as a few more people, who I though were completely new people in my life, but they weren't. They were, as in her brother's case, not new, but HER altered. Turns out, she had my husband's child (before I met my husband) but it didn't stop there. She had fallen in love with my husband way back when, and continued to stalk him, using disguises, and inevitably, he feel for it. They slept together on several occasions over a span of 40 years, though I didn't know that until recently. DOOR SLAM SHUT!
Now, tell me, if I didn't know what was going on because of the nature of the deception, how was i suppose to "talk to them about my concerns". I had established healthy boundaries early on. I have always had very good boundaries when it comes to the real world. But then, I think the situations I've encountered are not your normal, everyday situations so maybe this has nothing to do with what you're saying.