I suppose this counts as an USL incident.
In 1948 Sikorsky Hoverfly Mk 1 KK978 was conducting a trial on behalf of the Airborne Forces Experimental Establishment. It was (inocently) flying around its base at RAF Beaulieu advertising an upcoming airshow by towing a 13 foot long banner on a 50 foot long weighted rope, when the rope was struck by Spitfire NH840. The Hoverfly landed OK but the Spit was Cat 5.
Would this count as an Air to Air kill? and if so, was it a first for a rotorcraft?