在uiwebview里，如果load一个url还没结束就立即load另一个url，那么就会callback didFailLoadWithError method，error code is -999。solution is 在didFailLoadWithError 里添加下列code
if ([error code] != NSURLErrorCancelled) {
//show error alert, etc.
}ref link:
http://stackoverflow.com/questions/1024748/how-do-i-fix-nsurlerrordomain-error-999-in-iphone-3-0-os