In some circumstances, on Android it may happen that the icon of our apps providing badge notifications (eg. Facebook, WhatsApp, stock programs like Email or Messaging) remains stuck on a specific notification number, such as “1” as shown in the picture, even if there are no real notifications to be read:

In these cases, the quickest solution is to proceed as follows (the procedure may vary slightly depending on your Android version, the screenshots are related to an old Samsung Galaxy S2):

1. Open Settings->Application manager
   
2. Select “All” tab
3. Scroll until you find “BadgeProvider” system app
4. Click it and then click on “Clear data” in the bottom right part of the screen
   
5. Restart your device

In this way, the issue should be fixed. Note that on some more recent versions of Android, BadgeProvider may not be present.