ACR X NON ADHERENCE TO ATC INSTRUCTIONS MADE TURN BACK ONTO ACTIVE RWY WITH ACR Y CLRED FOR TKOF. PLTDEV SYS ERROR.

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AS LCL CTL IN CHARGE OF BOTH RWY 8L AND 8R I WAS MODERATELY BUSY. ACR X LANDED RWY 8L AND WAS ISSUED DETAILED TAXI INSTRUCTIONS TO CROSS RWY 8R. ACR X ACKNOWLEDGED THESE INSTRUCTIONS. AS LCL CTLR FOR THE DEP RWY; I ISSUED 'TAXI INTO POS' INSTRUCTIONS TO ACR Y; WHICH WAS STILL SEVERAL HUNDRED FT FROM THE APCH END OF THIS RWY. THE CREW ACKNOWLEDGED THE INSTRUCTIONS FOR TAXIING INTO POS AND HOLD. SCANNING THE RWY TO ENSURE THAT ACR X WAS CLR GAVE INDICATION TO ME; AS THE LCL CTLR; THAT ACR X WAS COMPLYING WITH INSTRUCTIONS. ACR X WAS IN CTR OF RWY WITH THE NOSE STILL AT 90 DEG ANGLE TO RWY. TAXIWAY WAS IN FULL VIEW TO THE CREW. I THEN OBSERVED ACR Y TAXI ONTO RWY 8R AT AN INTXN. THIS SEEMED VERY UNUSUAL; HOWEVER; IT CREATED NO PROBLEM. FROM THE TIME ACR X WAS OBSERVED PROGRESSING ACROSS THE RWY IN A NORMAL FASHION TO THE TIME ACR Y WAS ACTUALLY IN POS FOR DEP WAS ABOUT 1 MIN. I PROCEEDED TO ISSUE DEP CLRNC TO ACR Y WHEN I OBSERVED UNUSUAL MOVEMENT THROUGH MY PERIPHERAL VISION. ACR X HAD MADE A HARD 90 DEG TURN AND WAS TAXIING ON THE RWY. THIS ONLY PROVES A CTLR'S JOB IS NEVER COMPLETED. WE MUST WATCH EVERY ACFT AND BE AWARE OF THE PLTS WHO MAKE MISTAKES.