In The Crucible why do Reverend Samuel Parris and Judge Danforth fear being wrong?

The Reverend Parris and Judge Danforth have, in the course
of the play, condemned, arrested and accused hundreds of people in the town of Salem of
being witches.  Those people are in jail; numerous others have been hanged, and their
court decisions have made news and spread throughout the country.  Danforth holds a
proud court, meaning, one that takes pride in being right and in issuing edicts that are
obeyed and powerful.  He takes pride in his "logical" and supposedly fail-proof ways of
judging people.  We see this especially in act three when John, Giles and Francis
challenge his decisions on the accused.  He explains over and over again in different
ways how witchcraft works, how judging the bewitched works, and is so proud that he
refuses to acknowledge that they have any sense in what they are saying.  So, it is
evident that he is a proud man.

Add to this Reverend
Parris, who upon the play's beginning, expressed himself to feel like he was
underappreciated and disliked in the town. He felt he deserved more credit and fame. 
When the judges come in town and he becomes a very helpful and respected member of the
court system, his ego is finally stroked.  Also, it allows him to see many people he
didn't like be accused of witchcraft--there is a sense of sweet justice in that.  To top
it all off, he is afraid that his household will be seen as being one that housed a
witch--it was his niece, Abby, after all, that had started all the chaos with the
dancing in the woods.  He was terrified that his name would be associated with
witchcraft.  So, his zealousy in helping the courts helps to alleviate that
possibility.

Then, add to that the following:  if they were
wrong in judging all of those people, then their careers are over--especially
Danforth's.  He won't ever be trusted to judge anyone again. So add to the list fear of
losing your job or reputation.  Lastly, at the end of the play, they hear that a
neighboring town of Andover had overthrown the courts and that chaos had erupted in the
town. If they are seen as wrong, then they are afraid that the people of the town will
rise against them and do them harm.  After all, Parris already had a knife put in his
door.  Who knows what else the townsfolk would do!  They could be in danger of being
killed!  So, fear of their own lives is another factor.

I
hope that those thought helped; good luck!