Why does dispose mysteriously give me Invalid Pointer Operation errors?

The following source explains more specifically what happens with this error:

   program tnt03; uses crt;

     { Demonstration of pointer heap leaks, in a ludicrous,
       but easily seen example }

     type
       intptr = ^integer;

     var
       a, b: intptr;
     begin
       new(a);
       new(b);
       a^ := 2;
       b^ := 5;
       writeln(‘Note here that pointer a’ ‘s contents is ’, a^);
       writeln(‘and pointer b’ ‘s contents is ’, b^);
       writeln(‘When the pointer addresses are exchanged, and not the ’,
               ‘contents,’);
       a := b; {address location of pointer a to location of pointer b}
       b := a; {inactive, position of pointer a is already pointer b}
       writeln(‘the result is that the contents of pointer a is ’, a^);
       writeln(‘and the contents of pointer b is ’, b^);
       writeln(‘This is not desired, as both pointers point to the ’);
       writeln(‘same address on the heap, one pointer’ ‘s allocated ’);
       writeln(‘memory with new() is lost. It can happen ’);
       writeln(‘inadvertedly, but in most cases where this occurs, ’);
       writeln(‘it can not be easily seen. A position saved in heap ’);
       writeln(‘memory should always be removed with dispose. Because ’);
       writeln(‘this error occurs in this program, the first dispose ’);
       writeln(‘issued (a) will work properly, but the second dispose ’);
       writeln(‘will result in the runtime error listing you see
       writeln(‘below: ’);
       highvideo;
       writeln(‘Runtime Error 204: Invalid Pointer Operation’);
       writeln; writeln;
       normvideo;
       dispose(a);
       dispose(b);
       {position with 5 in it is disposed with first call, so 2nd call
        results in Runtime Error 204 Invalid Pointer Operation }
     end.

5.4. References

Gillin, P. 1995. Give Kahn credit. Computerworld. 29(5):36.

Wirth, N. 1996. Recollections about the development of Pascal. In T. J. Bergin and R. G. Gibson (Eds.), History of Programming Languages. Reading, MA: Addison-Wesley.