Extension conflict

Extension conflicts were sometimes a problem on Apple Macintosh computers running versions of Mac OS prior to X, especially System 7. Extensions were bundles of code that extended the operating system's capabilities by directly patching OS calls, thus receiving control instead of the operating system when applications (including the Finder) made system calls. Generally, once an extension had done what it wanted, it was supposed to pass on the (possibly modified) system call to the operating system's routine. If multiple extensions want to patch the same system call, they end up receiving the call in a chain, the first extension in line passing it on to the next, and so on, until the last extension passes to the operating system. If an extension does not hand the next extension in line what it is expecting, bad things result, ranging from unexpected behavior to full system crashes.