C3P0: Tried to check-in a foreign resource!

Earlier today I encountered the following stack trace whilst using C3P0 and Hibernate:

com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool - An Exception occurred while trying to check a PooledConection into a ResourcePool.
com.mchange.v2.resourcepool.ResourcePoolException: ResourcePool [BROKEN!]: Tried to check-in a foreign resource!
	at com.mchange.v2.resourcepool.BasicResourcePool.checkinResource(BasicResourcePool.java:657)
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$ConnectionEventListenerImpl.doCheckinResource(C3P0PooledConnectionPool.java:636)
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$ConnectionEventListenerImpl.connectionClosed(C3P0PooledConnectionPool.java:630)
	at com.mchange.v2.c3p0.util.ConnectionEventSupport.fireConnectionClosed(ConnectionEventSupport.java:55)
	at com.mchange.v2.c3p0.impl.NewPooledConnection.fireConnectionClosed(NewPooledConnection.java:510)
	at com.mchange.v2.c3p0.impl.NewPooledConnection.markClosedProxyConnection(NewPooledConnection.java:381)
	at com.mchange.v2.c3p0.impl.NewProxyConnection.close(NewProxyConnection.java:1246)
	at org.hibernate.connection.C3P0ConnectionProvider.closeConnection(C3P0ConnectionProvider.java:92)
	at org.hibernate.jdbc.ConnectionManager.closeConnection(ConnectionManager.java:474)
	at org.hibernate.jdbc.ConnectionManager.aggressiveRelease(ConnectionManager.java:429)
	at org.hibernate.jdbc.ConnectionManager.afterTransaction(ConnectionManager.java:316)
	at org.hibernate.jdbc.JDBCContext.afterNontransactionalQuery(JDBCContext.java:268)
	at org.hibernate.impl.SessionImpl.afterOperation(SessionImpl.java:444)
	at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1153)
	at org.hibernate.impl.QueryImpl.list(QueryImpl.java:102)

Apologies for the scroll bars… but hopefully this is indexed neatly by Google to help others!

After a bit of investigation it turned out this was due to a multi-threading issue and the solution can be summarised quite neatly:

never shut down Hibernate before all threads have finished what they’re doing (if they are using Hibernate)

Otherwise, you may get the above exception (or an equally unhelpful exception).