ISSUE SUMMARY
The default index format for 2dsphere and text indexes has changed between versions 2.4 and 2.6. If a user creates a 2dsphere or text index in 2.6, downgrades to 2.4, and subsequently tries to add, modify or remove an indexed document, it can corrupt the index or return incorrect results.
USER IMPACT
Users who downgraded from 2.6 with an existing text or 2dsphere index may corrupt these indexes or receive incorrect results when querying the corresponding collection(s).
SOLUTION
Run a check on startup to determine if any incompatible indexes are present, and exit if this is the case.
WORKAROUNDS
Users who have created a text or 2dsphere index under 2.6 and need to downgrade to 2.4 should drop the index before the downgrade, and re-create the index under 2.4.
AFFECTED VERSIONS
All versions from 2.4.0 to 2.4.9 are affected.
PATCHES
The fix is included in the 2.4.10 production release.