I live by the principle where I just won't say anything incriminating or extremely embarrassing if there is a mic or phone that could possibly be active. Little embarrassing stuff is fine, but for everything else, I assume the mic is hot even if I just muted it and I can see the mute indicator.

In addition to protecting against technical failures like you describe, I also don't get in the habit of saying stuff that one day I could accidentally say on a blatantly hot mic or in front of someone else.

The only mute button I really trust is the one inside my head.