System Maintenance

General Recommendation

Quote from Arch Wiki

Partial updates/upgrades are unsupported

Arch Linux [Manjaro] is a rolling release distribution. That means when new library versions are pushed to the repos, all developers and packagers, rebuild all packages against the libraries. For example, if two packages depend on the same library, upgrading only one package might also upgrade library (as a dependency), which might then break the other package which depends on an older version of the library.
That is why partial upgrades are not supported. do not use pacman -Sy [package] or any equivalent such as pacman -Sy followed by pacman -S [package] . Always upgrade (with pacman -Syu ) before installing a package. Be very careful when using IgnorePkg and IgnoreGroup for the same reason. If the system has locally installed packages (such as AUR packages), users will need to rebuild them when their dependices receive a soname bump.
If a partial upgrade scenario has been created, and binaries are broken because they cannot find the libraries they are linked against, do not "fix" the problem simply by symlinking. Libraries receive soname bumps when they are not backwards compatible. A simple pacman -Syu to a properly synced mirror will fix the problem as long as pacman is not broken.
The bash script checkupdates, included with the pacman package, provides a safe way to check for upgrades to installed packages without running a system update at the same time.

Act on alerts during an upgrade

When upgrading the system, be sure to pay attention to the alert notices provided by pacman. If any additional actions are required by the user, be sure to take care of them right away. If a pacman alert is confusing, search the forums and the recent news posts for more detailed instructions.

Deal promptly with new configuration files

When pacman is invoked, .pacnew and .pacsave files can be created. Pacman provides notice when this happens and users must deal with these files promptly. Users are referred to the Pacman/Pacnew and Pacsave wiki page for detailed instructions.
Also, think about other configuration files you may have copied or created. If a package had an example configuration that you copied to your home directory, check to see if a new one has been created.

Manjaro Recommendation

In addition to the above mentioned points, Archlinux has only one branch, Manjaro has three.

Please follow these Manjaro recommendations:

No temporary branch changes. Do not mix in packages from other branches.
After changes to branch and mirror list do a full system update

   sudo pacman -Syyu

Before installing new packages do a full system update

   sudo pacman -Syu [some-package-name]

Do not downgrade using the pacman downgrade switch (-Suu).

Additional information

Manjaro Forum

How bad can it be? User story

Arch Linux Wiki