A bug in software that cannot be duplicated while under observation in a debugger. German theoretical physicist Werner Heisenberg showed that observing quantum mechanical behavior actually alters it. A software debugger may sometimes change the operating environment just enough to perform the process without error. For example, data that might have been stored in hardware registers when the program was optimized may be stored in main memory when compiled for debugging, and the memory values may be less accurate.
Timing can also be a cause of Heisenbugs; for example, a multithreaded process may crash under normal operation but work perfectly when instructions are executed one at a time. See bug