Comment 11 for bug 930839

This error is also an assertion being hit in libxcb, rather than a segfault, so it doesn't imply a bug in the library but rather that the library is catching a bug from higher up in the stack. The error message is suggesting it could be an issue in how threading is done; making XCB calls from multiple threads can trigger such problems for instance. This may have the same root cause as bug #930004.