This is probably because you are accessing your JMS session using more than one thread concurrently.

JMS sessions are intended to be single threaded.

Only very few operations (like close) are permissible to be called from a different thread.

See the JMS 1.1 specification for more details.

Also see http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4016222