A vivid imagination
A sixteenth-century scholar, one Nicolas Cardan, saw on awakening one morning the "sun shining though shutters, showing dancing flecks of dust. Imagining he saw a monster in the dust biting off heads with its bloody fangs, he panicked, jumped out of bed and fled the house in only his shirt". (p. 62)
Nothing new about public hysteria then.
From Apocalypses: Prophecies, Cults and Millenial Beliefs through the Ages, E. Weber, Hutchinson, London, 1999, p. 62.