There is a lot of discussion online about this asserts.

Fact is that they prohibit a large number of Java and Qt4 apps from starting
up and that 99% of this apps are working fine when this asserts are disabled..

 - Clifford

--- ./src/xcb_xlib.c.orig	2009-01-20 20:56:25.000000000 +0100
+++ ./src/xcb_xlib.c	2009-01-20 20:57:10.000000000 +0100
@@ -38,7 +38,7 @@
 void xcb_xlib_lock(xcb_connection_t *c)
 {
     _xcb_lock_io(c);
-    assert(!c->xlib.lock);
+    // assert(!c->xlib.lock);
     c->xlib.lock = 1;
     c->xlib.thread = pthread_self();
     _xcb_unlock_io(c);
@@ -47,7 +47,7 @@
 void xcb_xlib_unlock(xcb_connection_t *c)
 {
     _xcb_lock_io(c);
-    assert(c->xlib.lock);
+    // assert(c->xlib.lock);
     assert(pthread_equal(c->xlib.thread, pthread_self()));
     c->xlib.lock = 0;
     pthread_cond_broadcast(&c->xlib.cond);