[antlr-interest] what's up with the error detect? confused!

[niry]

As heard of Antlr's great error recovery and support, I moved from Flex/Bison to here.
But, very confused to me, when I have this rule:

line : LPAREN WORD RPAREN

and when I supply this string "Helloworld", no error was detected! and I tracked into the line()
I saw if look head token is LPAREN match will continue; if not, no error just goto some loopxx.
So how can I detect such errors?
Any way, sorry for my poor english
Thanks for reply