"No-limit game. First player to act bets 6000.
Next player announces All In for 6500."

The additional $500 is not a raise.

Everyone after the $6500 all in bet, can raise except the original $6000 better. If all call back to him, he can only call the $500.

You had a raise coming.

Any all in bet in no-limit that is not equal to the original bet is not a raise.