We've all done it
I put someone on hold once to complain about them to my manager, and even though the LED on the hold button lit up, it did not mute my mic.
Forunately, I didn't call them names or anything, just complained about how slow and unhelpful they were, but she didn't say anything, and just emailed my manager after the conversation. We all got "retrained" after that. And I got a new phone :-)