AN ACR B737 CLRED TO CROSS RWY 19L AND HOLD SHORT OF RWY 19R SAW A PA28 ON FINAL AND HAD THE FO QUESTION THE CLRNC TO CROSS THE RWY. ATCT LCL CTLR CONFIRMED THE CLRNC AND STATED THE PA28 WOULD GAR. AFTER THE B737 CROSSED THE RWY; HE SAW THE PA28 LAND ON THE L RWY VERSUS A GAR. WHEN RPTR QUESTIONED THE TWR; THE CTLR SAID THAT THE PA28 WAS SUPPOSED TO; BUT DIDN'T. CONVERSATION WITH FACILITY MGR LATER CONFIRMED THE INCIDENT WAS BEING LOOKED INTO.

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I WAS THE CAPT OF ACR X; SCHEDULED FROM SNA-LAS. WE WERE GIVEN INSTRUCTIONS FROM GND CTL TO PUSH FROM GATE X; N ON TXWY A N OF INTXN L. AFTER DISCONNECTING THE TUG; WE CALLED FOR TAXI ON FREQ 120.8; AND WERE TO MAKE THE 90 DEG R TURN AND HOLD SHORT OF RWY 19L AT TXWY L. WE CHANGED FREQ TO 125.8 AND CONTACTED THE TWR. THE TWR CTLR CLRED US TO CROSS RWY 19L AND HOLD SHORT OF RWY 19R. WE CHKED FINAL APCH AND SAW A WHITE SINGLE ENG ACFT ON FINAL APCH FOR RWY 19L. I DID NOT MOVE THE ACFT BUT ASKED THE FO TO CONFIRM THE CLRNC BECAUSE THE PA28 ON FINAL APCH TO RWY 19L APPEARED TOO CLOSE-IN FOR LNDG ON RWY 19L. THE FO ASKED THE TWR CTLR TO CONFIRM THE CLRNC TO CROSS RWY 19L. THE TWR CTLR DID SO AND CONFIRMED WE WERE CLR TO RWY 19L AND STATED THAT THE ACFT ON FINAL APCH WOULD 'GAR.' AGAIN; I DID NOT MOVE THE ACFT UNTIL WE SAW THE NOSE OF THE SMALL ACFT 'PITCH-UP' AS THOUGH IT WERE STARTING A GAR. SINCE HE APPEARED TO BE DOING AS THE TWR HAD TOLD US HE WOULD DO; I THEN PROCEEDED TO TAXI ACROSS RWY 19L AND STOPPED AT THE HOLD SHORT LINE AT RWY 19R. I GLANCED MY VISION TO THE L AS I WAS SETTING THE PARKING BRAKE AND SAW THAT THE SAME PA28 WHICH HAD BEEN ON FINAL HAD JUST COMPLETED A LNDG ON RWY 19L. I THEN ASKED THE TWR THAT MY UNDERSTANDING WAS THAT THE PA28 ON FINAL WAS 'GOING AROUND.' THE TWR CTLR AGREED AND SAID THAT 'HE WAS SUPPOSED TO.' THE INCIDENT OCCURRED AT XA14Z. I DO NOT KNOW THE CALL SIGN OF THE PA28; BUT IT WAS A WHITE SINGLE ENG PIPER PA28. THE INCIDENT WAS RPTED TO THE SNA ATCT MGR AND IS CURRENTLY BEING INVESTIGATED BY THEIR 'SYS SPECIALIST; ETC.' IT IS MY BELIEF THAT MANY FACTORS CONTRIBUTED TO THE ATC ERROR. FIRST IS THE HIGH DENSITY OF TFC AT SNA; PARTICULARLY THE CLOSELY SPACED FAST TFC APCH FOR SMALLER TRAINING ACFT. SECONDLY WAS THE FACT THAT THE TFC PATTERNS ARE CTLED BY SEPARATE FREQS; MAKING IT DIFFICULT TO HEAR ALL INSTRUCTIONS AND CLRNCS GIVEN TO ALL ACFT INVOLVED IN THE TFC PATTERNS.