OS X: If you don't have permissions to write to /Library/Application Support/Unity, license file is simply not written and no message is shown to user.

1. Delete existing /Library/Application Support/Unity/Unity_v5.x.ulf file
2. Revoke access to /Library/Application Support/Unity:
2a. cd /Library/Application Support/Unity
2b: sudo chmod a-w .
3. Start Unity
4. Enter your serial in activation window and click OK/Start Unity buttons

Result: Fails to write the license file and doesn't provide a message to user