This was what I experienced a while ago. I worked on my macbook on the dinner table, and plugged the power adapter to the closest wall outlet in the kitchen. In a few minutes, I noticed that the battery percentage turned very low. It was not charging at all, but discharging. I turned the macbook around, and found the back was really hot. I then shut it down.
I asked Google what could be the problem, and found this. I followed the instruction to reset SMC. But it did not solve the problem.
I found the outlet was a GFCI outlet. It has a green led and two buttons. I suspected the outlet could be the problem, and plug the power adapter to a normal outlet. The problem was fixed. I still do not know if it was caused by that outlet, or all GFCI outlets are not safe for the macbook adapter.