Well, in NGUI this error can happen if you have too many things in one panel. Remember, panels batch everything into one draw call, which means a hard limit of 65k vertices (less on the mac I believe). You basically need to split up whatever you have in there into separate panels in order to draw what you need.
Unity is finally going to be adding 32-bit indices support in a future version based on the latest hackweek project mentioned on twitter -- but until then you need to split up your geometry.