Use this tag for all questions which appear to be a bug and should be reported to upstream project developers.
It is useful to allow possible bug questions because Stack Exchange gives more visibility to the related potential developer community. Once confirmed, a link to the bug tracker should be added. For questions tagged with bug a valid answer is a link to the upstream bug tracker since there is no real answer but upgrading the tools or libraries after the bug was fixed.
For more details, please refer to the meta discussion here: How to deal with questions which appear to be bugs?