why I have Crash???

program Project1;

{$APPTYPE CONSOLE}
{$H+}

const Key: array [1..35] of string = ('AND', 'ARRAY', 'BEGIN'
, 'CASE', 'CLASS', 'CONST', 'DIV', 'DO', 'ELSE', 'END', 'FOR'
, 'FUNCTION', 'IF', 'IMPLEMENTATION', 'INTERFACE', 'MOD'
, 'NOT', 'OF', 'OR', 'PROCEDURE', 'PROGRAM', 'RECORD'
, 'REPEAT', 'SHL', 'SHR', 'STRING', 'THEN', 'TO', 'TYPE'
, 'UNIT', 'UNTIL', 'USES', 'VAR', 'WITH', 'WHILE');

var S, St: String;
    Chi, I, J, T: LongInt;
    Num, Tochka, Comment, Stri: Boolean;
    Ch: Char;

function PosAfter(Sub, S: String; C: LongInt): LongInt;

var T: LongInt;

begin
  delete(S, 1, C);
  T:=Pos(Sub, S);
  if T<>0 then
    PosAfter:=C+T
  else
    PosAfter:=0;
end;

Function UpperCase(S: String): String;

const ChUp: String = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
      ChDown: String = 'abcdefghijklmnopqrstuvwxyz';

var I, T: LongInt;
    Te: String;

begin
  Te:=S;
  for I := 1 to Length(S) do
  begin
    T:=pos(Te[I], ChDown);
    if T<>0 then
    begin
      Te[I]:=ChUp[T];
    end;
  end;
  UpperCase:=Te;
end;

begin
  while not seekeof do
  begin
    readln(S);
    I:=0;
    if Comment then
    begin
      T:=Pos('}', S);
      if T>0 then
      begin
        Comment:=False;
        insert('</span>', S, T+1);
        I:=T+length('</span>');
      end
      else
      begin
        writeln(S);
        Continue;
      end;
    end;
    if Stri then
    begin
      T:=Pos('''', S);
      if T>0 then
      begin
        Comment:=False;
        insert('</span>', S, T+1);
        I:=T+length('</span>');
      end
      else
      begin
        writeln(S);
        Continue;
      end;
    end;
    while I<=Length(S) do
    begin
      Inc(I);
      if (Num) then
      begin
        if (S[I] in ['0'..'9', '.']) then
        begin
          if (S[I]='.')and(not Tochka) then
          begin
            Tochka:=True;
            continue;
          end;
          if S[I]='.' then
          begin
            if S[I-1]='.' then
            begin
              insert('</span>', S, I-1);
              inc(I, length('</span>'));
            end else
            begin
              insert('</span>', S, I);
              inc(I, length('</span>'));
            end;
            Tochka:=False;
            Num:=False;
          end;
          continue;
        end;
        if S[I-1]='.' then
        begin
          insert('</span>', S, I-1);
          inc(I, length('</span>'));
        end else
        begin
          insert('</span>', S, I);
          inc(I, length('</span>'));
        end;
        Tochka:=False;
        Num:=False;
      end;
      if (Chi>0) then
      begin
        if (Chi=1)and(S[I] in ['0'..'9']) then
        begin
          insert('<span class=string>', S, I-1);
          inc(I, length('<span class=string>'));
          Chi:=2;
          continue;
        end;
        if (not (S[I] in ['0'..'9'])) then
        begin
          if Chi=2 then
          begin
            insert('</span>', S, I);
            inc(I, length('</span>'));
          end;
          Chi:=0;
        end;
      end;
      if S[I]='{' then
      begin
        insert('<span class=comment>', S, I);
        inc(I, Length('<span class=comment>'));
        T:=PosAfter('}', S, I);
        if T>0 then
        begin
          insert('</span>', S, T+1);
          I:=T+length('</span>');
          continue;
        end
        else
        begin
          Comment:=True;
          break;
        end;
      end;
      if S[I]='''' then
      begin
        insert('<span class=string>', S, I);
        inc(I, Length('<span class=string>'));
        T:=PosAfter('''', S, I);
        if T>0 then
        begin
          insert('</span>', S, T+1);
          I:=T+length('</span>');
          Continue;
        end
        else
        begin
          Stri:=True;
          Break;
        end;
      end;
      if S[I]='#' then
      begin
        Chi:=1;
        Continue;
      end;
      if (copy(S, I, 2)='//') then
      begin
        insert('<span class=comment>', S, I);
        S:=S+'</span>';
        Break;
      end;
      if (Chi=0)and(S[I] in ['0'..'9'])and(not Num) then
      begin
        if not Num then
        begin
          insert('<span class=number>', S, I);
          inc(I, length('<span class=number>'));
          Num:= True;
        end;
        continue;
      end;
      for J := 1 to Length(Key) do
      begin
        St:=copy(S, I, length(Key[J]));
        if (UpperCase(St)=Key[J])and((I=1)or
          (not (S[I-1] in ['a'..'z', 'A'..'Z'])))and
          ((I+length(Key[J])=length(S))or
          (not(S[length(Key[J])+I] in ['a'..'z', 'A'..'Z'])))
          then
        begin
          insert('</span>', S, I+length(Key[J]));
          insert('<span class=keyword>', S, I);
          inc(I, Length('<span class=keyword>')+length(Key[J])+length('</span>'));
          break;
        end;
      end;
    end;
    writeln(S);
  end;
end.

URAAAA I have AC!!! Crash was because I>length(S)!!!!!