Never delete the Install cache folder %windir%\installer to free up some disk space. This will cause serious problems during service pack upgrades!
In one of my case, It happened. Installer folder was deleted and when I tried installing the Service pack 2 , It scolded me with this error:
"The detection failed, this can be due to a corrupted installation database."
Found these articles to collect the Install cache from other servers and update the failed server:
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