Only superuser can change binlog format. There is nothing wrong with a superuser being able to change format of the current session. The warning is an appropriate result.

But if we change binlog format , then we wont be able to flashback . And I think this is a bug ,because
If flash back is on we should not be able to change the binlog_format. And if this is allowed for session
binlog_format , how it is usefull ?

There are many things that won't work if a superuser changes something else, that's why these permissions are only given to a superuser. Flashback is not anyhow special, it's a small piece of very limited and rather obscure functionality. binlog format is much more important and widely used. Maybe the super user needs to make a change which doesn't need to be flashback-capable, but does require a statement format (a bulk update on a table without PK, if you are looking for an example).

Why does it not worry you that any DDL which any user (not even a superuser) can issue at any time might well totally break flashback? It's not a good enough reason to forbid DDL, right?

Flashback, on the other hand, can only be enabled/disabled at server startup, which means that if it puts a hard restriction on something, the server needs to be restarted every time to avoid this restriction. In a busy environment it's highly undesirable and often impossible.

oh.. got it , bug closed.