• Confirmed this is a problem with running a brew command and just brew installing one or more formulae?
• Ran brew update and retried your prior step?
• Ran brew doctor, fixed all issues and retried your prior step?
• Ran brew config and brew doctor and included their output with your issue?

I'm trying to install this font cask formula, but homebrew cannot properly detect the nested container. Note: I've confirmed this is an issue with homebrew, not cask.

I've determined that the issue is a result of cabextract writing warnings (in addition to errors) to stderr. Homebrew attempts to validate the cab file with .stderr.empty? which prevents the correct container type from being detected, and thus throws an error.

When the test is run:

# Write only stderr to stdout
$ cabextract -t -- ~/Library/Caches/Homebrew/Cask/font-microsoft-office--latest/ppviewer.cab 2>&1 >/dev/null
/Library/Caches/Homebrew/Cask/font-microsoft-office--latest/ppviewer.cab: WARNING; possible 5968 extra bytes at end of file.

but the file is actually valid, with no errors, and returns a clean exit code:

$ cabextract -t -- ~/Library/Caches/Homebrew/Cask/font-microsoft-office--latest/ppviewer.cab
 …
 All done, no errors.