mind the gap

A poem complete in its conception is necessarily compromised in composition.

made out of mist

A poem accomplished however implausible in its conception.

mindlocked

The poem you had in mind must often remain there.

death mask

XIII. The work is the death mask of its conception.

—Walter Benjamin, "One-Way Street," translated by Edmund Jephcott, Selected Writings, Volume 1: 1913-1926, M. Bullock and M. W. Jennings, eds. (Belknap Press, 1996)