Since Millfork is only a middle-level programming language and attempts to eschew runtime checks in favour of performance,
there are many situation when the program may not behave as expected.
In the following list, "undefined value" means an arbitrary value that cannot be relied upon,
and "undefined behaviour" means arbitrary and unpredictable behaviour that may lead to anything,
even up to hardware damage.
* array overruns: indexing past the end of an array leads to undefined behaviour
* stray pointers: indexing a pointer that doesn't point to a valid object or indexing it past the end of the pointed object leads to undefined behaviour
* reading uninitialized variables: will return undefined values
* when using modifying operators: calling non-pure functions in the left-hand-side index expression (like in `a[f()] += b`).
Currently, such functions may be evaluated either once or twice. This might be fixed in the future.
* when using modifying operators: calling functions on the right-hand-side index expression than modify any of the variables used on the left hand side