1 File Failed To Validate And Will Be Reacquired

If the problem persists, your local download cache might be corrupted.

This message becomes a concern only if:

There are several common reasons why a file might fail validation: 1 file failed to validate and will be reacquired

But then, the scan finishes and gives you a cryptic message: If the problem persists, your local download cache

In simple terms, Steam compared the files on your hard drive to the "master version" on its servers and found a discrepancy. If the problem persists