What will happen when run package between different ROS distributions?

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This works most of the time, provided the messages exchanged between nodes have not changed. Let's say you have message type A on fuerte on the "/whatever" topic and try to send it to a node in a groovy environment. If message type A for groovy is exactly the same, everything should work. If OTOH there has been a change to the message (let's say a std_msgs/Bool has been added to it for some reason) you will get a error complaining about the md5 sums for messages not being the same.

I'm not aware of other caveats right now and have been using a mixed fuerte/groovy setup extensively in the past few months.