We've had an Echo since before they were available to the public (ex-Amazon employee here), and it does occasionally beep when it thinks it hears "Alexa."

But it's very rare that it goes beyond that; usually we hear the beep and one of us will yell "never mind!", and then the Echo will go silent. Sometimes it will cancel itself, too. I think that the always-listening part is less discriminating than the active voice recognition, and that it can re-parse the last couple of seconds and decide, in retrospect, that you didn't say "Alexa."

That said, I just recently said to my wife, "I wonder if Alexa knows 'sudo make me a sandwich'". The timing was such that it parsed "sudo make me a sandwich" and answered, "Well, if you ask like that, how can I refuse?" :)

We had a good laugh. Funny thing was, looking at the app later, it was actually parsed as "Pseudo make me a sandwich." I bet Google Now would have corrected it to sudo. :)