It is illegal to assign a value to the for loop control variable inside the for loop. 

If the purpose is to leave the loop prematurely, use a break or goto statement.

program Produce;

var
  I: Integer;
  A: array [0..99] of Integer;
begin
  for I := 0 to 99 do begin
    if A[I] = 42 then
      I := 99;
  end;
end.

In this case, the programmer thought that assigning 99 to I would cause the program to exit the loop.

program Solve;

var
  I: Integer;
  A: array [0..99] of Integer;
begin
  for I := 0 to 99 do begin
    if A[I] = 42 then
      Break;
  end;
end.

Using a break statement is a cleaner way to exit out of a for loop.