ZID CTLR EXPERIENCED OPDEV WHEN FAILING TO HEAR INCORRECT READ BACK ON ASSIGNED DESCENT ALT.

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I GAVE THE PLT A CLRNC TO CROSS 15 MI S OF DECEE AT 12000 FT; WHICH HE READ BACK PERFECTLY. ABOUT 4 MI FROM THE XING POINT; THE PLT ASKED TO VERIFY THE XING POINT. I TOLD HIM IT WAS 15 MI S OF DECEE. WHEN I SWITCHED HIM TO APCH; HE SAID 'YOU DID GIVE US EIGHT; RIGHT?' I TOLD HIM IT WAS '120' AND ASKED HIM TO SAY ALT. I HAD TFC BENEATH HIM AT 10000 FT. THE PLT RPTED LEVEL AT 11000 FT CLBING BACK UP TO 12000 FT. AFTER LISTENING TO THE TAPE I LEARNED THAT WHEN THE PLT ASKED TO VERIFY THE MILEAGE; WHICH WAS LENGTHY AND CONFUSED; HE SLIPPED IN THESE LAST 2 WORDS: 'AT EIGHT.' AFTER REVIEWING THE RADAR INFO; IT WAS CLR THAT HE LEVELED OFF AT 12000 FT FOR 5 UPDATES AND THEN STARTED DOWN. IF A PLT IS EVER QUESTIONING A CLRNC; I WOULD MUCH PREFER THEM TO DOUBLE- OR TRIPLE-CHK THAN NOT. I COULD HAVE SOLVED THE PROB IF I WOULD HAVE RESTATED THE WHOLE CLRNC. I DIDN'T BECAUSE WE HAVE HAD SOME RECENT ERRORS IN WHICH THE CTLR RESTATED CLRNCS INCORRECTLY. IN THE END THE CTLR IS ALWAYS TO BLAME. OUR EMPLOYER EXPECTS PERFECTION; BUT WE ARE ALL HUMAN BEINGS. I EXPECT PLTS TO BE HELD ACCOUNTABLE FOR THEIR PART; BUT IT RARELY SEEMS TO HAPPEN. I TURNED THIS ONE IN MYSELF BECAUSE I THOUGHT IT WAS AN UNSAFE SITUATION AND THEY THANKED ME WITH AN OPDEV IN MY FILE AND THIS NASA FORM.