Hi Al -
Neither "return" nor "event disable" will cause LTM to send a RST.
(The "reject" command may be used to send a RST.)
From what I am able to discern, each event runs as a TCL procedure, and "return" terminates the procedure and thus terminates processing for that event until it is triggered again, while "event disable" has a wider effect, preventing the specified event from being triggered at all for the remainder of that connection. (Anyone who knows better, please correct me if I'm wrong about that...)
In this case, the preceding rule would have already sent the redirect, and the request is no longer available for header insertion by the second rule, thus the error.
If you were seeing RSTs, they may have been generated as a result of the TCL error.
HTH
/deb