What, no three way call faux pas?
I'm sort of surprised that nobody has offered up this one: it was common to "tap" someone into a call, making it a three-day call. And by "tapping" again you usually dropped the third party.
But not this time. We were in a meeting, calling anther office, when it came up that we needed some information from a co-worker who was not particularly fit for her job. Tapped her on, asked the question and got a response, then boss tapped to drop her from the call and said something to the effect of "man I hate working with her she is such a pain to work with", only to hear a response of "hey Bob I'm still on the line". Much backpedaling ensued.
From then on it was determined that it was better to hang up completely and redial the first call then to risk getting caught with the third party still engaged.