5. Any Historical Religious Event
I've learned throughout the years as of late, any discussion of religion can turn into vile anger. No matter what spiritual path someone chooses, someone else is going to be offended. This is a subject Doctor Who should stay clear of altogether. Just to make the vast audience happy, and to keep tempers cool. The last thing Doctor Who should do as a show is lose some of their fans because of the way they may write a particular story.