One of them being when Steam prompts an error after you validate Steam files and the file doesn’t get reacquired.
This is a very common problem with a lot of fixes available.
In many cases, the installed mods for a game may conflict with your Steam client and cause you unexpected errors.
Like the error states, it found something that it shouldn’t or the mods caused a file to be changed.
A manifest is a list of files present on your computer.
The manifest on the Steam servers contains the list of all the files which are supposed be there according to the most recent update.
If the manifests don’t match, Steam declares that there is an error and one (or more) files failed to validate and will be reacquired.
After every update, it requires that you restart the client properly.
Or there can be some incorrect configuration with your client and a complete restart would fix it.