ConcurrentModificationException

what is ConcurrentModificationException?if i'm modifying a list twice, and java notices that, why wouldn't java just let me do it?
[146 byte] By [Airwolfa] at [2007-11-27 2:00:49]
# 1
Stop crossposting!!! http://forum.java.sun.com/thread.jspa?threadID=5164045
KathyMcDonnella at 2007-7-12 1:40:04 > top of Java-index,Core,Core APIs...