A schrödinbug is a bug that manifests only after someone reading source code or using the program in an unusual way notices that it never should have worked in the first place, at which point the program promptly stops working for everybody until fixed. The Jargon File adds: “Though… this sounds impossible, it happens; some programs have harbored latent schrödinbugs for years.”
The name schrödinbug was introduced in the version 2.9.9 of the Jargon file, published in April 1992. It is derived from the Schrödinger’s cat thought experiment. A well-written program executing in a reliable computing environment is expected to follow the principle of determinism, and that being so the quantum questions of observability (i.e., breaking the program by reading the source code) posited by Schrödinger (i.e., killing the cat by opening the box) affecting the operation of a program is unexpected.