Dudley Henriques wrote:

Got a friend in the business who relates that this scene was not even in the
script. Ford was fooling around with the actor swinging the sword; pulled
the gun and shot him. Story is that Spielberg absolutely broke up laughing;
immediately stopped filming long enough to put the scene in the script and
reshot it the same day.

I read that this scene was a result of Harrison having food poisoning the
day of that scene's shoot.

IMDB has a similar tale in their trivia section of the movie:

http://www.imdb.com/title/tt0082971/trivia

(scroll down to the bottom and it is two up from the last entry)

--
Peter