Something I see all the time is people having problems with their Facebook app, Skype app or many others. This happens both on iPhones and iPads as well as Macs. Many people update their OS software through the Apple Software Updates but they often don't update the apps themselves.
This can lead to all sorts of problems, because the newer versions of the apps generally add more functionality and fix bugs that have cropped up in previous versions. The older versions of the app may not be compatible with the version of the OS that you're using.
So, the best strategy is generally to keep all your apps up to date, unless you know of a good reason not to. Look for the little badge beside the App Store icon that shows how many updates are waiting to be installed.