So CBS aired a special promising "TV's Greatest Surprises" (as determined by a Nielsen Entertainment survey).

Fortunately, here's a list of their top 30 of TV's Greatest Surprises - so you don't have to spend all night actually watching them! (Towards the end, it just seemed like one surprisingly grisly death scene after another.)
 
Though they scoured through 60 years of television, if you believe their list, only four of TV's most surprising moments happened prior to the 1980s.

1950s: Lucy fights with an Italian woman while crushing grapes on I Love Lucy

1960s: Twilight Zone "surgeries" episode
       Baby mix-up on The Dick Van Dyke Show

1970s: Premiere of All in the Family
       Announcement of the death of Colonel Blake on M*A*S*H