The light dawns - public synonym called SYS - how to drop?

With reference to my other post on various SYS packages being unavailable, I now know the cause. Despite my declarations of confidence, it turns out that one of our guys (a student on work placement as it happens) wanted to create a public synonym and called it "SYS".

2 things

1. am surprised that Oracle (10gR2) didn't object to this
2. we are unable to drop the synonym

16:25:05 SQL> drop public synonym sys;
drop public synonym sys
*
ERROR at line 1:
ORA-04098: trigger 'XDB.XDB_PI_TRIG' is invalid and failed re- validation

Elapsed: 00:00:00.18
16:25:58 SQL> Anyone know how to tidy up this mess?

--
jeremy