The WordPress Core code is updated pretty regularly throughout the year. Everyone wants to upgrade to the latest version of the WordPress as it is stable. We all want the latest features and also immunity from the security holes of the older versions. Yet, even after such extensive research, all the bugs cannot be fixed at once.
This is where the problem arrives. Those bugs which left behind while fixing can really cause a lot of trouble. Also, there are plugins and themes which don’t act nicely in an update. The core developer of plugin and themes need some time to catch up with an update of WordPress. So, till that time, things can go weird.