This wiki page tells :

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."

I've no idea what they are talking about. Can someone provide an example of how it is like (like with a fictional situation)?