Data definition commands, like creating or dropping databases, collections or indexes, will fail on a secondary if that secondary has a background index build in progress on the same collection.
The failure is silent and can lead to inconsistency between replica set members.
Building the index in the foreground avoids the issue. If users rely on background index builds, they should confirm that all index builds have completed before running any data definition commands.
The secondaries wait to apply the data definition commands if a background index build is in progress.
Version 2.6.0 is affected by this bug.
The patch is included in the 2.6.1 production release.