Your perfect spec sitcom script is your only shot  at becoming the next Diane English, and yet many smart would-be writers make the same stupid mistake. Instead of writing a typical Frasier using only the Frasier characters, the naive newbee creates an  unusual outside character, who drives the story.  This is NOT what the writer-producer is looking for!  Wise up.  It could be two years before your next  script gets to the top of the reading pile (and by then you may be too old).  Don't waste your big break!
This is no-no # 1.