Something that happened to me shortly after updating my Macbook Air to Mountain Lion was that on waking up from sleep, Networking would crash. Crash to the point of having to hard boot my Air to get everything back to normal. I thought this was odd, so to the goog I went! Luckily, I wasn’t the only one with this problem.
The problem seems to occur mostly after an upgrade. I say that, because the fix that I found here worked for me.
>Go to “System Preferences > Network” and select the WiFi-Service from the list on the left. Select
“Options” “Advanced” from the bottom right of the right pane. In this window, delete all the known Networks and hit OK. Then go ahead and delete the WiFi device. Create a new WiFi device and you’re done.
After you delete the WiFi device, press apply before creating a new WiFi device. Once done, click Ok and you should be good to go. To test, close the lid on your air or put your Mac to sleep, count to 10 and open the lid or wake it up by press the Any Key on your keyboard.
So far I’m finding Mountain Lion to be really nice. I like the iOS style notifications. I like the sharing features. I like the Twitter integration (and can’t wait for the Facebook integration). I like the new features in Safari. And I really like the Airplay screen mirroring with the Apple TV.