While upgrading a cluster to a newer version, I upgraded the Cluster Master first. and then pushed out a bundle. Immediately after pushing a bundle, the indexers crashed with this :
splunkd: /home/build/build-src/galaxy/src/clustering/CMBundleMgr.cpp:766: bool CMSlaveBundleMgr::findOrDownloadBundle(bool&, Str&): Assertion `bundleId == bundleChecksum' failed.
See SPL-135257 @ http://docs.splunk.com/Documentation/Splunk/6.5.1/ReleaseNotes/Knownissues#Indexer_and_indexer_clust...
hidden files can cause bundle checksum mismatches between versions. either dont push a bundle with hidden files while in mixed-version clustering (for some specific versions), or don't put hidden files into a bundle!