Consider following snippet:

#include <bits/stdc++.h>
using namespace std;

int main() {
    int x=3;
    int y=1;
    int z=2;
    cout<<(&x)<<' '<<(&y)<<' '<<(&z)<<endl;
    set<reference_wrapper<int>> a;
    a.insert(ref(x));
    a.insert(ref(y));
    a.insert(ref(z));
    for(const auto& i: a) cout<<i<<' '<<endl;
    y=10;
    for(const auto& i: a) cout<<i<<' ';
    return 0;
}

What would happen to underlying container when a property used by container for sorting is modified?

Edit1 : Given the answer I posted that ordering can go wrong, is it a bad practice to use reference_wrapper inside stl containers then (specially where mutations can cause container to invalidate its assertions)?