Years ago after my 2nd attunement I went home and gave Reiki to my now ex husband. After a few minutes he squealed pushed my hands away from his forehead and said "!%%^&^& you burnt my head!!! you need to learn to control this!!" I thought he was joking - but within minutes a bright red mark appeared on his forehead that looked like sunburn - it lasted about 24 hours and went away.
I asked my master about it and she said she had never heard anything like it. Nothing similar ever happened when I give treatments since - but never doubt the intensity of energy a person can draw. I thought maybe that I had not had time to adjust to the attunement. Plus my ex had been through a bad accident as a teenager that left him permanently disabled - so maybe he needed more than many people and because we were very close he was wide open to the maximum flow I could channel?
I did give him reiki many times after that but was always very mindful to put a big distance between my hands and the first hand placement and kept checking "okay?" "too warm?".. etc
The funny thing was he was a skeptical engineer thinking it was "the power of suggestion" until the experience - after that he didn't doubt there was something real going on...