This is a friendly word of warning! Don't call etree.clear_error_log() from multiple threads. We found this issue during stress tests of our CherryPy based application. Every worker thread was calling etree.clear_error_log() after the page was rendered. Apparently we hit some sort of race condition. Backtrace: C [etree.so+0x21977] C [etree.so+0x22d48] C [libxslt.so.1+0xdc17] xsltTransformError+0xf7 C [libxslt.so.1+0x839d] C [libxslt.so.1+0xa5da] xsltParseStylesheetProcess+0x80a C [libxslt.so.1+0x1efbc] xsltParseStylesheetInclude+0x1ac C [libxslt.so.1+0xa0b4] xsltParseStylesheetProcess+0x2e4 C [libxslt.so.1+0xb4b9] xsltParseStylesheetImportedDoc+0x1e9 C [libxslt.so.1+0xb5b8] xsltParseStylesheetDoc+0x28 C [etree.so+0xe640c] C [python2.5+0x4ff83] C [python2.5+0x11e67] PyObject_Call+0x27 Christian