Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
How To Use Notification Centre in Mountain Lion
Notification Centre has been an integrated feature on Apple’s mobile devices for a year now since its release for iOS 5 in October 2011 and has helped users worldwide keep on top of their different alerts and reminders, as well as anything else that they need to be updated on. Notification Centre collects it all in one place. Now Mac users can take advantage of it too with its inclusion with OS X Mountain Lion. But useful as it is, it may take some getting used to, so here are some pointers.
First you need to configure it.
To do this, go to System Preferences and select Notifications. You will see a list of applications that you can individually configure.
Click on an app and you will be presented with three choices for presenting them. None speaks for itself, Banners appear on the top right of the screen for a few seconds and Alerts pop up until you close them.
There are also three check boxes providing further options.
Show in Notification Centre has a tab next to it that will allow you to select how many recent items will be displayed when a notification pops up. You can choose from 1, 5, 10 or 20 most recent items to be shown.
Badge app icon will make a number appear above an application’s icon when it has notifications for you, instead of making them flash up automatically so you can check them at will instead of being distracted.
Play sound when receiving notifications does exactly what it suggests and alerts you with a noise every time you are notified of something, so you can be kept alert when you are away from the computer.
Although it can be time consuming, Notification Centre gives you the flexibility to go through every application and set each one to notify you the way you want.
To access Notifications Centre, click on the far-right icon on the toolbar at the top of the screen, which looks like a bulleted list. It will slide into view and present all your registered notifications.
A nice feature included with Mac’s Notification Centre is the Share Button which integrates Twitter updates into your desktop, so you can update it directly from the sidebar.
If you wish to take advantage of this, click on the Share Button icon in Notification Centre and check the box that says Show share button in Notification Centre. You will also need to be signed in Twitter for it to work.
Once this is set up, you need to open Notification Centre and click the Click to tweet button, which will bring up a Tweet Sheet.
As is the case with Twitter, you will be able to type up to 140 characters here, as well as add your location if you so desire. Once you are satisfied with it, click Send and it will be posted straight to your Twitter account.
Notification Centre is fairly simple to use and you will be able to tweak it to update on as much or as little of your Mac’s activities as you like, even turning them off completely if they prove too distracting.