The Jargon File (version 4.4.7, 29 Dec 2003):

schroedinbug
 /shroh'din?buhg/, n.

    [MIT: from the Schroedinger's Cat thought-experiment in quantum physics] A
    design or implementation bug in a program that doesn't manifest until
    someone reading source or using the program in an unusual way notices that
    it never should have worked, at which point the program promptly stops
    working for everybody until fixed. Though (like bit rot) this sounds
    impossible, it happens; some programs have harbored latent schroedinbugs
    for years. Compare heisenbug, Bohr bug, mandelbug.