This term is properly spelt "QUALITY" (without the quotes), in ALL CAPS. This tag is written in lowercase due to limitations in Danbooru's tagging system.
The term "QUALITY" indicates (with ironic intent) glaring yet humorous errors and flaws in commercial works of poor quality or works of otherwise good quality. This can be caused by poor budgetting (blowing most of the budget of an anime into the first episodes, per example), a small budget or outsourcing to cheaper studios, although it can also be an honest mistake. A classic example is Yoake Mae Yori Ruri Iro na's QUALITY cabbage.