Some other factors, which are beyond the control of the WordPress include:
-indiscriminate use of plugins by the users.
-indiscriminate use of themes by the users.
-indiscriminate tweaking of the theme's templates.
-messing with the WordPress "Core Files".
-Not reading/following Documentation.
-Not seeking professional help when necessary.
-using webhosts or servers not equipped to install and run WordPress properly.
-Restrictions/limitations of servers.
-Not properly validating websites to resolve markup and CSS errors.
-Not willing to learn with patience.
-Expecting everything in a ready-made package free.
-Moving WordPress or changing the URL without following the codex guides