Solr should log the error message(and warning) because in production server it's impossible to see the console output.

(Correct me if there is one existing already..)

Message is like this.

Jan 12, 2012 5:03:54 PM org.apache.solr.common.SolrException log
SEVERE: org.apache.solr.common.SolrException: ERROR: [935ae61e5233e6d675a50293a9efc589] multiple values encountered for non multiValued field attr_title_t: [fdsadfsadfsadkf;jsf;jsafljdfsfasfasf]
at org.apache.solr.update.DocumentBuilder.toDocument(DocumentBuilder.java:243)
at org.apache.solr.update.processor.RunUpdateProcessor.processAdd(RunUpdateProcessorFactory.java:60)
at org.apache.solr.handler.XMLLoader.processUpdate(XMLLoader.java:147)
at org.apache.solr.handler.XMLLoader.load(XMLLoader.java:77)
at org.apache.solr.handler.ContentStreamHandlerBase.handleRequestBody(ContentStreamHandlerBase.java:55)
at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:129)
at org.apache.solr.core.SolrCore.execute(SolrCore.java:1360)
at org.apache.solr.servlet.SolrDispatchFilter.execute(SolrDispatchFilter.java:356)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:252)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1212)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:399)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:766)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:450)
at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230)
at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:945)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.jetty.bio.SocketConnector$Connection.run(SocketConnector.java:228)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

Until now I haven't found anywhere.

java/logs only log requests.