I’ve ruined more than one video in my time by forgetting to turn the external mic on.
That’s why I was delighted the other day when I hooked an external mic to a Panasonic GH2 and was shown this message (or similar; I don’t have the camera with me right now):
Be sure the external microphone is turned on.
Nice design. The right message at exactly the right time.
I guess I’d probably want an option somewhere down in the advanced settings to not display that message, so that folks who don’t need it can turn it off (on that note, I wonder if the camera is smart enough to not bother with the message if you’ve got headphones plugged in so you can monitor the audio), but for this user at least, the message reminded me of the value of delivering a succinct, useful message at the exact moment of need.