pattern match and substitution, can you help?

file named test.txt
I want to replace all the words Event with the word Fatal in all lines containing the word ERR - but I also want to keep the output of the other lines not matching ERR


Test.txt:
Event 13 INF egegegege
Event 14 INF egegegege
Event 15 ERR egegegege
Event 16 MSG egegegege

The output I want it:
Event 13 INF egegegege
Event 14 INF egegegege
Fatal 15 ERR egegegege
Event 16 MSG egegegege

Doesnt appear to work - any ideas?

bash-2.05# cat test.txt
Event 13 INF egegegege
Event 14 INF egegegege
Event 15 ERR egegegege
Event 16 MSG egegegege

bash-2.05# sed -e '/ERR/{s_Event\(.*\)_Fatal\1_g}' test.txt
sed: command garbled: /ERR/{s_Event\(.*\)_Fatal\1_g}

Aha - I knew there must be a relatively simple way.. I am using sed but not well up on this and was not sure of the syntax..

Thanks for all your help!!