When Vern is telling the others about how he heard about the dead body when he was under the front porch, One guy suggests reporting to the police about the body through an anonoumous call. The other guy doesn't want to because they can trace calls he saw it on Dragnet. That movie takes place in 1959. Dragnet wasn't around until the mid-1960's.
If you check "The Complete Directory To Prime Time Network And Cable TV Shows", you will find the Dragnet started as a radio show in 1949 and moved to television January 3, 1952. It left the air in September of 1959, then returned in September of 1967 and aired until September of 1970. So this is not a slip-up at all.