We had the issue that some of the xml documents which we were parsing caused a crash on iOS 6. The fix is to add the following line to the AQXMLParser line 1141:

p->serror = __errorCallback2;

And adding the following method to the same file:

static void __errorCallback2(void *userData, xmlErrorPtr error) {

}

The main problem was that the serror field is not initialized to any thing valid so the call stack you get when it is crashing is more or less random.
Our crashes were also only happening when we were parsing on a background thread.