Problem Description: What is the problem?
What did you expect to happen: Why do you think this is an issue?
What happened instead: How is what happened different from what you expected?
Why is this bad/What are the consequences: Why do you think this is an important issue?
Steps to reproduce the problem: The most important section. Review everything you did leading up to causing the issue.
When did the problem start happening: If your report is about something that used to work but no longer does, when was the last time you remember it working?
Possibly related stuff (which gamemode was it? What were you doing at the time? Was anything else out of the ordinary happening?): Anything else you can tell us.