Broadcast outlets, which often repeat stories they read in the paper without doing any fact-checking, are also especially vulnerable to April 1 hoaxes.

FORBES: April Fool's turns into March Media Madness

Do I seriously have to merely repeat myself, and encourage you to read not only primary sources but your own remarks?

FORBES: Virginia's Pre-Abortion Ultrasound Law Medically Unsound-Violates Guidelines of American College Of Obstetricians And Gynecologists

At the Sunday night vigil, which opened as the sun went down, Mr. Hickenlooper read the names of all 12 victims, asking the crowd to repeat "We will remember, " after each name.

WSJ: Services, Vigil Remember Victims