Forgetting to silence your phone when you should know better is something I think we can all sympathize with. It happens to all of us. If you have a Pebble watch and an Android device, changing the current sound profile from your wrist is possible.
In order to silence your phone from your watch you'll need to download and install a free app from the Play Store. The app, Pebble Phone Ringer Switcher, is small and requires no setup.
After installing the app on your Android device, launch it and tap on the "Install PBW to Watch" button. The Pebble app will then launch, installing the Phone Ringer app on your watch.
After installing both apps, one on your phone and one on your watch, go to the main menu on your Pebble watch. Scroll down until you find Phone Ringer and select it.
You'll see three small icons lined up on the right side of your watch screen. Each icon represents a different sound profile for your Android device. Pressing the top button on your watch will enable all sounds, or Normal Mode. The middle and bottom buttons will activate vibrate and silent modes, respectively.
The Play store listing states that the app has only been tested on Android 4.1, but it should work on Android 2.3 and above. I've successfully used it with Android 4.2.2 on a Nexus 4.