When a background index build is interrupted on a secondary, for example by dropping the collection during the build, the clean-up process can in some circumstances fail with an assertion and not clean up the incomplete index correctly. Further attempts to drop the collection fail because the index is still marked "in progress" and not cleaned up correctly.
See also related issue
Only background index builds on secondaries are affected.
Use foreground index builds, or ensure that the secondaries have finished building the index before dropping a collection.
Production release versions from 2.6.0 to 2.6.3 are affected by this bug.
The fix is included in the 2.6.4 production release.
The patch now ensures that IndexBuildBlock::fail() can never throw an exception.