Are there any cases where it is incorrect to replace push_back with emplace_back?

Question

Can I break a valid C++03 program by replacing std::vector::push_back with emplace_back and compiling it with C++ 11 compiler? From reading e

Answer 1

Yes, you can change the behavior (more than just avoiding a copy constructor call), since emplace_back only sees imperfectly forwarded arguments.

#include 
#include 
using namespace std;

struct Arg { Arg( int ) {} };

struct S
{
    S( Arg ) { cout << "S(int)" << endl; }
    S( void* ) { cout << "S(void*)" << endl; }
};

auto main()
    -> int
{
    vector().ADD( 0 );
}

Example builds:

[H:\dev\test\0011]
> g++ foo.cpp -D ADD=emplace_back && a
S(int)

[H:\dev\test\0011]
> g++ foo.cpp -D ADD=push_back && a
S(void*)

[H:\dev\test\0011]
> _

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Addendum: as pointed out by Brian Bi in his answer, another difference that can lead to different behavior is that a push_back call involves an implicit conversion to T, which disregards explicit constructors and conversion operators, while emplace_back uses direct initialization, which does consider also explicit constructors and conversion operators.