Part III of the poem "The Lady of Shalott" by Alfred, Lord Tennyson describes a curse on the Lady of Shalott. It states:
A curse is on her, if she stay
Her weaving, either night or day,
To look down to Camelot.
She can weave and look at the mirror to see the life outside her island, including the river flowing down to Camelot, but if she stops weaving to look directly at the river, she will trigger the curse.
One day, she sees Sir Lancelot in her mirror and goes to look out her window at him directly. This triggers the curse. As a sign of this, her mirror cracks. When she realizes what has happened, she goes outside and carves the name "The Lady of Shalott" on a shallow boat, launches it on the river, and floats down to Camelot. When she arrives there, she is dead.