Humans were superstitious about their reflections long before the invention of the mirror as we know it today. The ancient Greeks believed it was unlucky to see one's reflection, exemplified by the story of Narcissus, who fell in love with the sight of his reflection in a stream and died, pining, at the water’s edge [source: Britannica].
Despite Narcissus's tragic tale, the ancient Romans couldn't help but figure out how to see their reflections without trekking down a flat body of water. They used convex, highly polished metal surfaces as mirrors [source: Britannica]. These were highly prized possessions, which may explain why breaking a mirror was considered such bad luck.
The earliest mention of the broken mirror superstition in English is a 1777 account claiming that breaking a mirror is "a very unlucky accident" because mirrors were part of an "ancient kind of divination" and "formerly used by magicians in their superstitious and diabolical operations" [source: Brand]. Since then, broken mirrors have been popular in British culture, as seen in this poem by Alfred, Lord Tennyson:
She left the web, she left the loom
She made three paces thro' the room
She saw the water-flower bloom,
She saw the helmet and the plume,
She look'd down to Camelot.
Out flew the web and floated wide;
The mirror crack'd from side to side;
'The curse is come upon me,' cried
The Lady of Shalott.