For any users currently testing diamondback, there is a small chance
that rosbag might advise you to run the script: "fix_msg_defs.py".
This script is intended to repair bags that, due to a bug in an early
release of cturtle, are missing their message definitions.  The script
is currently broken and the fixed bag will have incorrect data for all
but the first instance of a particular message type.

The script is now fixed in trunk and will go out with the next release of ROS.

If you need to migrate such a bag, I recommend grabbing the copy of
fix_msg_defs.py from trunk.  Feel free to contact me if you have any
questions.

Sorry for the inconvenience,
--Jeremy