Ok I looked into the corresponding gcc stl function and it seems that the debug function checks if the iterator is incrementable. I haven't checked the macro that performs this check but the iterator might be marked as invalid after an element has been erased. So this crash will only happen during a debug build, but I think it is good to know that during a debug build some additional checks are performed. the relevant code is: _Safe_iterator& operator++() { _GLIBCXX_DEBUG_VERIFY(this->_M_incrementable(), _M_message(__msg_bad_inc) ._M_iterator(*this, "this")); ++_M_current; return *this; } I still don't understand why it fails with my patch but this might be because it is not delivered correctly. I'll push a patch based on your idea to master. <div class="gmail_quote">2011/6/21 Noel Power <<a href="mailto:nopower@novell.com" target="_blank">nopower@novell.com</a>> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div>On 21/06/11 18:52, Markus Mohrhard wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> I did a debug build. And I had some stl debug symbols in my backtrace so it might be that perhaps the compiler won't optimize this out. I attached the backtrace. print it shows: it = {<__gnu_debug::_Safe_iterator_base> = {_M_sequence = 0x29e3678, _M_version = 0, _M_prior = 0x0, _M_next = 0x0}, _M_current = {_M_current = 0x2a27bf0}} </blockquote></div> ah, ok -D_GLIBCXX_DEBUG is enabled, right I tried to build some stuff partially ( with some extra debug flags etc. ) <sigh> didn't work ( link problems ) I will try a full debug build ( have never done that up to now ) Noel </blockquote></div>