There has been at least one instance of an accident inside a multiply-occupied holding stack. Fortunately, none of the other aircraft in the stack became involved in the accident sequence.
On 8 December 1963, PA214, a 707-120 en route from San Juan to Philadelphia via Baltimore, entered a holding pattern with five other aircraft already in it due to a squall line hovering over Philadelphia. Sixteen minutes after entering the stack, the aircraft was struck by lightning, which caused a series of fuel-tank explosions, which caused the aircraft to crash. At least one of the five other aircraft in the stack saw the burning aircraft falling to the ground, but none of the five were actually struck by the accident aircraft or the detached debris therefrom.
To quote the CAB report:
[...] Flight 214 reported over the New Castle VOR at 2042, 5,000 feet [sic] and control was then transferred to Philadelphia Approach Control which provided the crew with the following:
“... Philadelphia weather, now, seven hundred scattered, measured eight hundred broken, one thousand overcast, six miles (visibility) with rain shower, altimeter two nine four five, the surface wind is two hundred and eighty degrees at twenty (knots) with gusts to thirty (knots). I’ve got five aircraft, have elected to hold until this ... extreme winds have passed, ... do you wish to be cleared for an approach or would you like to hold until the squall line ... passes Philadelphia, over?”
The crew advised Philadelphia they would hold and were instructed to hold west of New Castle VOR on the 270[°] radial and given an expected approach clearance time of 2110. The crew requested and received permission to use two minute legs in the holding pattern. At 2050:45 the crew advised Philadelphia they were ready to start an approach. They were told to continue to hold and they would be cleared as soon as possible. The crew acknowledged with “Roger, no hurry, just wanted you to know that ... we’ll accept a clearance.” Approximately eight minutes later, at 2058:56 the following transmission was heard on the Philadelphia Approach Control frequency 124.6 “MAYDAY MAYDAY MAYDAY ... Clipper 214 out of control. Here we go.” Seconds later another transmission on the same frequency was heard “Clipper 214 is going down in flames.” This latter transmission was made by the first officer of National Airlines Flight 16 (NAL 16). ... NAL 16 was in the same holding pattern as Flight 214 but 1,000 feet higher, and the first officer had seen the Pan American flight descending on fire. [Page 3; emphasis mine.]