The chines on the sides of the YF-12 fuselage were cut-back to just ahead of the cockpit to help the big Hughes air-to-air intercept radar work better and they also had an IR "eye-ball" sensor built into the leading edge - but apparently, these small modifications had a marked effect on the stability of the aircraft and made it noticeably more difficult to fly. The other A-12 variants (i.e. the A-12 itself and the SR-71A and -B) had the chines go right up to the tip of the nose.
One of my best friends (d. 2012) was on the original A-12 design team at Lockheed and while he didn't discuss very many details (me being a foreigner and with all that Canadian aluminium flooding across the border, a substantial security risk to the supply of beer cans in the United States), he did tell me a few funny stories about the aircraft.
Like the Lockheed U2/TR-1, each Blackbird aircraft was different and each had its own distinct quirks: some aircraft were very reliable and easy to deal with, while others were really quite miserable machines on which there was always a snag juuuust prior to launch.