Our library changes (not present in the CTP) will break some nonconformant programs. One popular example, occurring throughout VS's own codebase, is people using back_inserter() without including <iterator>. We changed our header dependencies (the Standard headers include each other in unspecified ways), and now <iterator> isn't dragged into some translation units where it was before.

Obviously, we're trying with all our might to avoid breaking conformant programs. But we depend on CTP and Beta feedback to identify as many occurrences of this as possible - as large as our test matrix and internal codebase are, they're dwarfed by our users' codebases.