Troubleshooting

Run brew update (twice) and brew doctor before creating an issue!

When creating a formula-related issue please include the link output by running:

brew gist-logs <formula>

Check for common issues

Run brew update (twice).
Run brew doctor and fix all the warnings (outdated Xcode/CLT and unbrewed dylibs are very likely to cause problems).
Read through the Common Issues.
If you’re installing something Java-related, maybe you need to install Java (brew cask install java)?
Check that Command Line Tools for Xcode (CLT) and/or Xcode are up to date.
If things fail with permissions errors, check the permissions of /usr/local's subdirectories. If you’re unsure what to do, you can cd /usr/local && sudo chown -R $(whoami) bin etc include lib sbin share var Frameworks.

Check the issue tracker to see if someone else has already reported the same issue.
Make sure you check issues on the correct repository. If the formula that failed to build is part of a tap like homebrew/science or homebrew/dupes check there instead.

If you had a formula error: run brew gist-logs <formula> (where <formula> is the name of the formula that failed to build).
If you encountered a non-formula bug: upload the output of brew config and brew doctor to a new Gist.

Create a new issue titled "<formula name> failed to build on 10.x", where <formula name> is the name of the formula that failed to build, and 10.x is the version of macOS you are using and including the link output by brew gist-logs