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

Jargon File

The Jargon File is a glossary of computer programmer slang. The original Jargon File was a collection of terms from technical cultures such as the MIT AI Lab, the Stanford AI Lab and others of the old ARPANET AI/LISP/PDP-10 communities, including Bolt, Beranek and Newman, Carnegie Mellon...

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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

Wine (software)

Wine is a free software application that aims to allow computer programs written for Microsoft Windows to run on Unix-like operating systems. Wine also provides a software library, known as Winelib, against which developers can compile Windows applications to help port them to Unix-like...

, which makes it possible to run many Windows applications on other platforms, needs to be bug compatible with Windows too.

A hardware example is found in the design of the IBM AT A20 address line

A20 line

The A20 or addressing line 20 is one of the plethora of electrical lines that make up the system bus of an x86-based computer system. The A20 line in particular is used to transmit the 21st bit on the address bus....

to emulate the behaviour in older processors.