BUF TRACON CTLR RESOLVED A DEVELOPING CONFLICT WITH 2 DEP ACFT.

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I HAD JUST COME TO WORK. THE BRIEFING CTLR TOLD ME THE C340 AT 4000 FT 'WAS DIRECT BUF VOR; THEN DIRECT IAG;' I THINK. JUST PRIOR TO BRIEFING CTLR UNPLUGGING; LCL CALLED TO 'APREQ' A TURN TO HDG 090 DEGS FOR DEPARTING BUF B190; CLBING TO 10000 FT. SINCE DIRECT BUF VOR FOR C340 WOULD KEEP C340 ABOUT 6 MI SE OF BUF ARPT; I APPROVED THE TURN. AS B190 WAS CLBING THROUGH 28000 FT; I OBSERVED C340 E OF BUF VOR APPROX HDG 320 DEGS TOWARD B190. I CALLED B190 TFC OUT TO C340; WHO REPLIED 'I HAVE HIM IN SIGHT.' AS I WAS DOING THIS; THE CONFLICT ALERT WENT OFF. I TOLD C340 TO MAINTAIN VISUAL SEPARATION; THEN TOLD B190 OF TFC AND B190 SAID HE SAW C340. AFTER THE SIT CLRED; I ASKED C340 IF HE WAS DIRECT BUF VOR. HE SAID NO; HE WAS DIRECT IAG; ABOUT A 310 DEG HDG. I THEN TURNED C340 TO 270 DEG HDG TO KEEP ACFT CLR FOR BUF ARPT DEPS. MISTAKE #1: NOT CONFIRMING C340'S ACTUAL CLRNC AND HDG; ESPECIALLY BECAUSE ACFT WAS CLOSE TO THE DEP CORRIDOR AND AT LOW ALT OF 4000 FT. MISTAKE #2: NOT JUST STOPPING DEP (B190) AT 3000 FT TO ENSURE SEPARATION. THAT'S MY JOB. I FAILED AND I WON'T GET CAUGHT LIKE THAT AGAIN. I MISTAKENLY BELIEVED THE 2 ACFT WERE NOT IN CONFLICTION BECAUSE OF DIVERGING HDGS. WRONG! MISTAKE #3: MY JUDGEMENT WAS BASED ON POTENTIALLY INCORRECT INFO DURING MY BRIEFING. I FAILED TO MAKE SURE THE INFO WAS ACCURATE; ESPECIALLY WHEN BRIEFING INCLUDED THE WORDS 'I THINK.' MISTAKE #4: YRS OF EXPERIENCE HELPED TO PREVENT A BAD SIT FROM GETTING WORSE. BUT THOSE SAME YRS SOMETIMES FOG THE PERSON'S JUDGEMENT TO GET THE SIT CORRECT. THE SIT WAS SOMETHING I SHOULD HAVE BEEN RIGHT ON TOP OF; NOT BEING NONCHALANT BECAUSE 'I'VE SEEN IT ALL.' I FELL INTO A TRAP; BUT SHOULD HAVE SEEN IT COMING. I ALWAYS TRY TO LEARN FROM MY MISTAKES; SO I DON'T REPEAT THEM. THIS WAS A DUMB MISTAKE ON MY PART.