Can you find this Delphi coding error (part 5)?

Can you spot the error in the following (somewhat artificial) Delphi code?

unit ResultCreator; interface implementation type TResultMode = (Worst, Bad, Good, Better, Best); function GetResult(Year : Integer) : TResultMode; begin .. end; procedure Process; const adjustment = 1; var Arr : array[0..3] of Integer; Idx : TResultMode; Elem : Integer; begin Arr[0] := 55; Arr[1] := 66; Arr[2] := 77; Arr[3] := 88; Idx := GetResult(2016); case Idx of Good : Elem := 2; Better : Elem := 0; Best : Elem := 1; else Elem := 3; end; Writeln(IntToStr(Arr[Elem-Adjustment])); end; end.

This was quite easy, right?

Solution: This code will at least sometimes result in an runtime error, when the local variable Elem contains the value 0. The array index for Arr on the last line of code will then be invalid (-1).

Not so hard to spot! But imagine finding this problem among thousands of lines of code spread over hundreds of units! And consider also a code base that is evolving all the time, so the code must constantly be rechecked.

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