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Computer hardware or 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.
An aspect of maintaining backward compatibility with an older system is that such systems' client programs often do not only depend on their specified interfaces but also bugs and unintended behaviour. That must also be preserved by the newer replacement. Besides the significantly higher complexity that needs to be maintained during the natural evolution of the code or interface, it can sometimes cause performance or security issues, and the inconsistencies in the behaviour of interfaces can sometimes lead to new bugs in the software using it, creating difficult to resolve multi-directional cross dependencies between various pieces of code.