If the language specification doesn't expressly say what happen when things "outside the design" happen, then different implementations may work differently.
If the language design spec says
"If an array index is out of bounds, exit the program and return a value of ABEND_ARRAY_BOUNDS_VIOLATION to the calling program,"
that may seem very specific, but if how to "exit the program and return a value of ABEND_ARRAY_BOUNDS_VIOLATION to the calling program," isn't specified by someone (usually the operating system), then it may not be specific enough. if different operating systems specify how to do this differently, then expected "under the hood" behavior will not necessarily be consistent across operating systems.
For example, does "exit the program" mean simply returning control to the caller, or does it mean explicitly returning any resources that were previously granted to the program by the operating system first? Or is that optional? If it's optional as far as the operating system is concerned, does the language provide a compile- or run-time switch to force such a cleanup? Does returning memory to the operating system guarantee that the OS will sanitize the memory, and if not, does the language guarantee it? If the language doesn't guarantee it, does the language provide a compile- or runtime switch so the program will sanitize memory prior to returning it to the operating system?
These differences in language implementations and even differences in how operating systems handle the starting and stopping of processes can lead to differences in what the code actually does. Usually these differences are unimportant but sometimes they are very important.