Proposition for WP:NOBOOMERANG (v2):

1) Posting to a noticeboard to report another editor opens you up to inquiry regarding any of your actions that are significantly related. There is no immunity for reporters, nor any immunity for statements made by anyone in the context of the report.
For example, if you report someone for incivility, it is entirely relevant whether you were also uncivil or baited them. In the same way, if you report someone for WP:3RR violation, any edit-warring you are currently engaging in on that article is relevant. If someone launches a false or baseless report, they may be sanctioned for it.
The end result is that bringing up a problem that you are part of may result in you being sanctioned, perhaps even to the exclusion of the other parties, as your actions may have created mitigating circumstances for them.
2) The initial complaint defines and limits the scope of the report. A report is not an opportunity to bring up unrelated matters, although if such a matter is not stale, it may still be reported elsewhere. However, that additional report must be made separately and placed in the appropriate forum, where it will be resolved independently. Note that a matter caused by a comment made in the report itself is handled as part of the report.
For example, if you report someone for incivility, it is not relevant whether you were yourself uncivil to someone else in a different article or at a different time. If you report someone for WP:3RR violation on one article, it is irrelevant whether you edit-warred on this article in the past or are edit-warring right now on some other article. Of course, the latter case would be a reasonable basis to file a separate report. If one of the participants were to hurl vulgarities, this would be dealt with right then and there, without a need to file anything separately.
The goal is to prevent reporting editors from having unrelated matters piled on top of the report to distract from its goals and to penalize participants. Counter-reporting unrelated incidents is permitted but strongly discouraged, as such reports will be viewed more skeptically with regard to motive.
3) While any editor is free to comment on a report, extreme discretion should be used with regard to speaking against an editor when there is a conflict of interest or history of disputes. You must identify any of these factors and be especially careful with regard to keeping comments within the scope of the report. Third-party comments that violate this must not be deleted, but but must not be replied to, and may be hatted or otherwise minimized if they prove to be distracting.
For example, if an editor with whom you are engaged in an content dispute files a complaint about someone else's behavior and you wish to participate in the discussion, you must state that you are in conflict with them and avoid making undue generalizations. If, after you report an editor for edit-warring, someone who you've had a conflict with comes in and starts posting complaints about your prior behavior, it is be considered out of scope.
The goal is to prevent outside editors from ganging up and importing drama.