Roman,

> IIRC, it can happen only when the application tries to close the already
> closed connection. Do you call XAConnection.close() method in your code?

No, definitely not. It must be called by JBoss internally!?
And it just happend again after a while without activity on my side:

2009-04-30 14:40:01,781 WARN [org.jboss.resource.connectionmanager.JBossManagedConnectionPool] (JCA PoolFiller) Unable to fill pool
org.jboss.resource.JBossResourceException: Could not create connection; - nested throwable: (java.lang.IllegalStateException: Cannot execute desired operation because pooled connection has invalid state.)
at org.jboss.resource.adapter.jdbc.xa.XAManagedConnectionFactory.getXAManagedConnection(XAManagedConnectionFactory.java:465)
at org.jboss.resource.adapter.jdbc.xa.XAManagedConnectionFactory.createManagedConnection(XAManagedConnectionFactory.java:409)
at org.jboss.resource.connectionmanager.InternalManagedConnectionPool.createConnectionEventListener(InternalManagedConnectionPool.java:611)
at org.jboss.resource.connectionmanager.InternalManagedConnectionPool.fillToMin(InternalManagedConnectionPool.java:527)
at org.jboss.resource.connectionmanager.PoolFiller.run(PoolFiller.java:74)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.IllegalStateException: Cannot execute desired operation because pooled connection has invalid state.
at org.firebirdsql.pool.AbstractPingablePooledConnection.checkValidity(AbstractPingablePooledConnection.java:176)
at org.firebirdsql.pool.AbstractPingablePooledConnection.internalClose(AbstractPingablePooledConnection.java:285)
at org.firebirdsql.pool.AbstractPingablePooledConnection.deallocate(AbstractPingablePooledConnection.java:302)
at org.firebirdsql.pool.PooledConnectionQueue.destroyConnection(PooledConnectionQueue.java:252)
at org.firebirdsql.pool.AbstractConnectionPool.getPooledConnection(AbstractConnectionPool.java:254)
at org.firebirdsql.pool.AbstractFBConnectionPoolDataSource.getPooledConnection(AbstractFBConnectionPoolDataSource.java:359)
at org.firebirdsql.pool.AbstractFBConnectionPoolDataSource.getPooledConnection(AbstractFBConnectionPoolDataSource.java:397)
at org.firebirdsql.pool.AbstractFBConnectionPoolDataSource.getXAConnection(AbstractFBConnectionPoolDataSource.java:431)
at org.jboss.resource.adapter.jdbc.xa.XAManagedConnectionFactory.getXAManagedConnection(XAManagedConnectionFactory.java:449)
... 5 more
2009-04-30 14:40:01,781 WARN [org.jboss.resource.connectionmanager.JBossManagedConnectionPool] (JCA PoolFiller) Unable to fill pool
org.jboss.resource.JBossResourceException: Could not create connection; - nested throwable: (java.lang.IllegalStateException: Cannot execute desired operation because pooled connection has invalid state.)
at org.jboss.resource.adapter.jdbc.xa.XAManagedConnectionFactory.getXAManagedConnection(XAManagedConnectionFactory.java:465)
at org.jboss.resource.adapter.jdbc.xa.XAManagedConnectionFactory.createManagedConnection(XAManagedConnectionFactory.java:409)
at org.jboss.resource.connectionmanager.InternalManagedConnectionPool.createConnectionEventListener(InternalManagedConnectionPool.java:611)
at org.jboss.resource.connectionmanager.InternalManagedConnectionPool.fillToMin(InternalManagedConnectionPool.java:527)
at org.jboss.resource.connectionmanager.PoolFiller.run(PoolFiller.java:74)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.IllegalStateException: Cannot execute desired operation because pooled connection has invalid state.
at org.firebirdsql.pool.AbstractPingablePooledConnection.checkValidity(AbstractPingablePooledConnection.java:176)
at org.firebirdsql.pool.AbstractPingablePooledConnection.internalClose(AbstractPingablePooledConnection.java:285)
at org.firebirdsql.pool.AbstractPingablePooledConnection.deallocate(AbstractPingablePooledConnection.java:302)
at org.firebirdsql.pool.PooledConnectionQueue.destroyConnection(PooledConnectionQueue.java:252)
at org.firebirdsql.pool.AbstractConnectionPool.getPooledConnection(AbstractConnectionPool.java:254)
at org.firebirdsql.pool.AbstractFBConnectionPoolDataSource.getPooledConnection(AbstractFBConnectionPoolDataSource.java:359)
at org.firebirdsql.pool.AbstractFBConnectionPoolDataSource.getPooledConnection(AbstractFBConnectionPoolDataSource.java:397)
at org.firebirdsql.pool.AbstractFBConnectionPoolDataSource.getXAConnection(AbstractFBConnectionPoolDataSource.java:431)
at org.jboss.resource.adapter.jdbc.xa.XAManagedConnectionFactory.getXAManagedConnection(XAManagedConnectionFactory.java:449)
... 5 more