I have a couple that comes immediately to mind.
First up, as another poster mentioned, having a workplace relationship. I was working in the transport industry at the time...
I was newly separated, he was older with a well-known drinking problem. In hindsight I think I did it as a bounce back from my marriage, either or, it made great gossip about the workplace. One day someone had an open niggle at me as to who I was seeing, and I told them my private life, was private. That brought gales of laughter from those who heard my comment. I felt about 5 cms tall, I hadn't realised until then the guy I was seeing was very loose-lipped after downing a few...
Secondly, also at the same workplace, I received a call from ''john's mum'' (name change of course) saying John's Dad had been rushed to hospital, could I let him know asap. All of the guys were in a union meeting and I called John out of the meeting to let him know. His face dropped, and he raced to the phone. Shortly after, John approached me to say it wasn't his Dad at all. It then dawned on me we had two John's at the workplace... it was just this one was the one who came to mind.
I profusely apologised for my mistake and the correct John was informed. When the union meeting found out about my mistake, the union delegate wanted to call a walkout and asked for my employment to be terminated. The first John was a very decent chap and stood up for me, he was happy with my apology and that it was a genuine mistake. They then tried to work on the second John to file a complaint as he ''should have been told sooner''.
Needless to say I felt terrible over the whole thing, but it was an honest error. I was eventually pleased to leave that company for other reasons. Knowing the married union delegate was having an affair with another workplace friend who used me as an excuse to her Hubby as to her absences from home made things tricky at times too.