This one is simple, but useful. :)
When compile a VIEW, the error "ORA-00911: invalid character" poped up.
AND f.COL_ABC not in
ERROR at line 444:
ORA-00911: invalid character
Copy above 2 "not in" into a text editor, you'll see the invisible Special Character.
TextWrangler can show the invisible Special Characters. I couldn't find this function in SubLime Text 2.
The solution is simple too: remove the string contains Special Character, and then type them again.