Scotland's top clinical director said Nicola Sturgeon's failure to wear a mask in a pub was "easily done".
Jason Leitch said the First Minister is "furious" with herself but defended the "little lapse" after a picture of her emerged.
The Scottish Sun published a photograph of Sturgeon talking to three people on her way out of the funeral wake of a close colleague.
Strict Scottish Government coronavirus rules state customers in hospitality venues must wear a face covering except when seated, and must wear one when moving around.
Sturgeon was attending a funeral wake for a Scottish Government civil servant.
She said: "Last Friday, while attending a funeral wake, I had my mask off briefly. This was a stupid mistake and I'm really sorry.
"I talk every day about the importance of masks, so I'm not going to offer any excuses. I was in the wrong, I'm kicking myself, and I'm sorry."
The Scottish Sun reported she was at the Stable Bar and Restaurant after attending a funeral at the nearby Mortonhall Crematorium.
In an interview with BBC Radio Scotland today, Leitch said: "I think the First Minister knows this matters, I think she's as furious with herself as some might be with her.
"We had a brief conversation last night and she’s absolutely mad at this little lapse in concentration.
"It's so easily done, we live in a completely different world from a year ago, don't we?"
He said Sturgeon was "just about out the door and was called back".
He added: "Her mask was off because she was leaving, and then (she went) back in and (had) a little lapse – didn’t put it back on.
"It just reinforces again to all of us the nature of these instructions and this virus, and she is as mad with herself as everybody else."
