Computer hardware and software is said to be bug compatible if it exactly replicates even an undesirable feature of a previous version. The phrase is found in the Jargon File.[1]
An unfortunate aspect of maintaining backwards compatibility towards an older system is that such systems' client programs often do not only depend on their specified interfaces, but also on bugs and unintended behaviour. This must also be preserved by the newer replacement.
Famous examples of this area can be found in Windows, which has traditionally emulated many old system bugs in order to allow older low-level programs to run. Wine, which makes it possible to run many Windows applications on other platforms, needs to be bug compatible with Windows too.[2]
A hardware example is found in the design of the IBM AT A20 address line to emulate the behaviour in older processors.