Agree. It should be **important** paragraph.
I investigated it and catched the issue with MySQL.
However I see where is problem:
`JdbcChannelMessageStore#pollMessageFromGroup` invokes `removeMessageFromGroup` and I suppose we have not to return anything if we haven't removed the `Message` from DB:
```
if (updated != 0) {
	logger.debug(String.format("Message with id '%s' was deleted.", id));
} else {
	logger.warn(String.format("Message with id '%s' was not deleted.", id));
}
```
or don't use that method and call DELETE directly and check the result.
I think I'll push it to see.