Have you ever picked up the phone and someone says something and from what they say and the sound of their voice, you think you know who it is? Notice I said think you know.
I had a major f***up yesterday by doing just this and making some smart ass remark, only to find out, of course, that it wasn't who I thought it was, but a public citizen calling me about an issue with my employer's website.
I didn't make some innocuous remark either, I said "what do you want jackass?". Oh my god, the silence on the other end immediately told me I'd made a mistake. I apologized profusely but the citizen was duly outraged and I'll bet I was red as a tomato. Luckily he accepted my apology and I was later able to help him with his issue.
This bothered me for the rest of day and I can gaurantee I won't be spouting off on the phone anymore. Turns out I was the jackass.