1. Always try to reproduce or play (when looking for feature) on your playground installation, that you can create using latest stable code + latest fixes.
If you are able to reproduce, then open issue on mantis
2. Search, search, search on all our communication channels (forum, linkedid, twitter) and on your favorite search engine, before posting or reporting an issue.
3. Read CHANGE LOG available with each TestLink distribution, is a good source of information regarding new features & issue fixes.
4. Read you favorite guideline for issue reporting before creating one, doing your best to provide as much as possible, details that can allow to reproduce same scenario.