The Republic of Krakow was odd, and many of the newly created German states went beyond odd, but overall, the Congress of Vienna was a pretty impressive feat. In an ideal world, they would have created an independent Poland and a confederation of Italian states that was separate from Austria, and given all those states constitutions and parliaments, but realistically, fgor what they actually could do, they did remarkably well. I can't think of anything that could be considered as a serious mistake by the lights of the time, except the fundamenntal dedication to legitimism, reaction and the predominance of the Russian-Austrian-Prussian troika. And that was a feature, not a bug.