MongoDB 3.0 secondaries running non-MMAPv1 storage engines (i.e.: WiredTiger, RocksDB) may fail to properly handle updates involving keys longer than 1024 bytes if the key is indexed.
Setting failIndexKeyTooLong to false has no effect.
A secondary receiving an update involving a key longer than 1024 terminates if there's an index on that key. If enough secondaries are affected the replica set may become unavailable.
It is highly recommended that users do not use keys longer than 1024. When upgrading from 2.6, please upgrade directly to version 3.0.3 or higher.
MongoDB versions 3.0.0, 3.0.1, and 3.0.2 are affected by this issue.
The fix is included in the 3.0.3 production release.