It started years ago when I was at school. My best friend throughout school was a bit of a frenemy to me, and would often engineer fall outs or would speak her mind to someone, and inevitably I would end up getting the blame or taking the flack for it, with people falling out with me, whilst it wouldn't have any effect on my friend and she would pull the old 'I'm not getting involved' line even though she'd stirred up trouble in the first place. I think she would often allude to others that I was involved or I thought the same as her too, or I'd said X or Y couldn't come along with us when I hadn't, and no one dared to fall out with her so instead muggins here took the flack.
Now it's continued into adulthood too: Two friends of mine, we'll call them Friend A and Friend B had a falling out. Friend A decided to stop talking to me when she stopped talking to Friend B. I phoned her a couple of times and tried to sort things out but she didn't want to know so I just thought 'stuff her' and stopped trying. Friend B then suddenly decides life is too short for grudges, and the next thing they are close again, and I am left on the sidelines despite having no involvement in their falling out whatsoever.
Also, and this is going to sound incredibly petty but it is another example: I met a group of women on a parenting forum. One woman and I decided to form a FB group for members of that forum to join. It was a support group really. The woman and I were admin but she seemed to have issues with a few other people that wanted to join and said that they couldn't join, and she told me that she'd spoken to them and been honest with them about why they couldn't join. Next thing these women have deleted me from FB but are still friends with the woman who let them join. So again it looks like I've taken the flack for that too, as she's clearly told them it was a joint decision or that I said they couldn't join, when it was her that had the issues with them.
These kinds of things happen to me all the time. I'm not the most assertive person ever but I'm not a wallflower either. I just get embroiled in these kinds of things and take the blame for no fault of my own.