CORPORATE PLT DSNDS BELOW ASSIGNED ALT.

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IN CRUISE FLT 90 NE OF PSK WHILE RESETTING RNAV AND ENJOYING SOLO LEG OF TRIP; CTR CALLED FOR ALT CHK. I REPLIED I WAS AT 13400. I WAS ASSIGNED 14000. CTR ADVISED NO CLRNC GIVEN. I ACKNOWLEDGED. I WAS OFFERED A DSCNT CLRNC BUT OPTED TO RETURN TO 14000. CTR 127.92 ADVISED OF GRAVITY OF SITUATION. I WAS AWARE OF IT. PROBLEM DEVELOPED WHEN I RESET THE RNAV SYS AND WAS COMPARING THEM. I PLACED A PACK OF CRACKERS ON THE CONSOLE. THE CRACKERS BUMPED THE ALT HOLD BUTTON ON THE AUTOPLT AND WHILE I ENJOYED THE OUTSIDE VIEW; THE ACFT SLOWLY DSNDED. THE ONLY EXCUSE WAS FAULTY XCHK ON MY PART AND ATTN OUTSIDE OF COCKPIT. WHEN 1 FUNCTION IS DELETED; THERE ARE NO BELLS OR FLASHING LIGHTS AS WITH AUTOPLT DISENGAGE. ACFT NOT EQUIPPED WITH ALERTER. WX WAS CAVU FROM 8000 UP; WITH A SCATTERED LAYER AT 8. AFTER 2 WKS OF RAINY WX; I LET MY ATTN WANDER DURING A CAVU LEG. WHEN IFR; FLY IT (IFR). NO CONFLICTS WITH TFC. CTLR ALMOST AS UPSET WITH ME AS I WAS. SINCE I WAS THE ONLY ONE IN THE ACFT; IT HAD TO BE MY FAULT. ONLY MITIGATING FACTOR WAS THIS WAS ONLY THIRD TRIP AS SINGLE PLT IN OVER A YR; GOT LAZY. SOME FATIGUE OR STRESS DUE TO PENDING JOB CHANGE. AFTER I REGAINED MY COMPOSURE; I ASKED THE CTLR IF THERE WOULD BE ANY PROBLEMS FROM THIS. HE REPLIED NOT FROM HIS END. THE INCIDENT WAS VERY DISTURBING TO ME; LONG AFTER IT WAS OVER; EVEN THOUGH THERE WAS NO CONFLICT. I THOUGHT I WAS TOO PROFESSIONAL TO RING A BELL (300 FT) LET ALONE 600 FT. PRO'S WORK DURING AND RELAX AFTER THE FLT; NOT DURING.