See attached or Gist for script to reproduce. Run against an empty sharded test environment as it is dropping data unexpectedly.
https://gist.github.com/882010

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Original report: http://groups.google.com/group/mongodb-user/browse_thread/thread/f3c0aee26fca75e9#
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The basic premise is simple, we have three databases with a "prefixed" name.
db2 = db.getSisterDB('fred_sharded') // sharded
db3 = db.getSisterDB('fred_sharded2') // also sharded
db4 = db.getSisterDB('fred_shardednot') // not sharded

If I drop db2, data from db3 also loses data:
db2.runCommand(

)

db4 (unsharded) seems unaffected