if the update (option 1) fails, the user is offered to forcefully install udpates (if a connection to the mirror server is present):
the part in the “help” page describing this is the second to last line (in front of "Attention: ") here:
1 - UPDATE SYSTEM "sudo pacman -Syu" "yaourt -Syua" The first command compares a list of all installed packages with package database on your system repository mirror/server. If an updated package is available from your system repositories, it will get downloaded and installed on your system. The second command does the same as the first part, but with one exception: It also downloads, compiles, and installs all packages from the Arch User Repository (AUR), which have an updated PKGBUILD file. The first command is still needed occasionally, because major Pacman updates require "pacman -Syu" and will fail when started with an AUR helper. If updates from system repositories fail, the user is offered the choice to update packages using "sudo pacman -Syu --overwrite A-Z,a-z,0-9,-,.,_". Attention: When a new version of an AUR package is available, sometimes the PKGBUILD file is not updated. If you want to install the latest version of a single AURpackage, (re-)install it with INSTALL PACKAGES. If you want to install the latest versions of ALL AUR packages use FORCE UPDATE AUR.
this has probably happened to you.
it is actually a feature.
old versions of pacui had “force update”, “force remove packages”, and “force install packages” options. the first 2 options got integrated in the “update system” (option 1) and “remove packages and deps” (option 4), in case these options fail.
“force install packages” was NOT integrated in another pacui option but completely removed, because of multiple reasons:
if the package installation fails, there is something fundamentally wrong with your system and you should fix it first.
whenever i just want to look at package info, i use “pacui i” (option 3) and instead of installing a package, i quit pacui with CTRL+C. if “force install packages” were integrated, i had to quit it as well. this is really uncomfortable.
i am currently using yay, too. but an update has not failed for me for a long time. this means, i cannot tell you yay’s behavior in this case.
but yay offers you to show PKGBUILD differences before you update an AUR package. maybe, you are talking about this?