xmlCleanupParser function in parser.c

/*...*/
xmlCleanupGlobals();
xmlResetLastError();
/*...*/

OS: Windows (but I think others too)
problem appears then I load my library on libxml in one thread and unload it in another thread.

xmlCleanupGlobals - cleans global variables.
xmlResetLastError have such code:

void
xmlResetLastError(void)
{
    if (xmlLastError.code == XML_ERR_OK) /* xmlLastError is macro!!! */
        return;
    xmlResetError(&xmlLastError);
}

here it is:
xmlError *
__xmlLastError(void) {
    if (IS_MAIN_THREAD)
	return (&xmlLastError);
    else
	return (&xmlGetGlobalState()->xmlLastError);
}

in the function xmlGetGlobalState() we create global variables again, if they are not created.
becouse of this I've got memory leak.

You should change order of the functions in xmlCleanupParser:
/*...*/
xmlResetLastError();
xmlCleanupGlobals();
/*...*/

Thank you for your work, and sorry for my english :-)