I think I found the cause. because one of the line is larger than the set limit, I tried to set the flume.event.max.size.bytes in the agent node and collector node but the system doesn't seem to take the values 
    <description>The length of line content in byte.</description>
am I doing anything wrong?

From: Huang, Zijian(Victor) [mailto:zijian.huang@etrade.com]
Sent: Monday, September 26, 2011 12:15 PM
To: flume-user@incubator.apache.org
Subject: Flume agent repeatedly streaming the same content forever (Version: 0.9.3, r)

   I have encountered this problem with Flume twice. Flume agent just keep sending the same log file again and again to the collector and filling up all the disk space in the collector host at the end. Do you guys know what exactly causes Flume to lost count of the lines and keep re-streaming. I saw it happen when I try to stream some binary logs, and I saw it happen today with normal logs(may contains some binary data). I can replicated the problem easily. I am using "tail" to stream the content over
Please let me know what are the potential causes.