ISO Pascal reference manual

*Execution error -- NIL pointer used
Appendix C
This message indicates that an attempt has been made to follow a pointer
which is currently NIL. As this means that the pointer is not pointing at
anything the operation would be meaningless.
PROGRAM error(output);
TYPE pi = !integer;
VAR a : pi;
BEGIN
END.
a := NIL;
a! := 123; (wrong}
writeln('Error not detected');
*Execution error -- disposing NIL pointer
This error is the result of calling DISPOSE on a pointer which has the value
NIL.
PROGRAM error(output);
VAR pi : !real;
BEGIN
END.
pi := NIL;
Dispose(pi);
writeln('Error not detected');
*Execution error -- reference to DISPOSED object
This message indicates that a pointer has been left pointing to the remains
of an object that has been disposed. This is usually the result of copying a
pointer variable and using the copy after DISPOSE has been applied to the
original.
PROGRAM error(output);
TYPE pi =Tinteger;
VAR original, copy : pi;
BEGIN
NEW(original);
original! := 123; (valid}
Pascal Issue 1 107