A few fixes have been added, but none of them have fully fixed it. At this point, it should be moved into noPassthrough.
mr_drop.js (via parallel/basicPlus.js) failing on Nightly Linux 64-bit
mr_drop.js (via parallel/basic.js) is failing on MCI windows_64
mr_drop.js can fail when its internal race condition is altered by other tests in the parallel suite