Function pointers are one scary area. I don't know if C++11/C++14 decided to fix this yet. Most people relied on undefined behavior to make this work.
Source: The Design and Evolution of C++, by Bjarne Stroustrup. (I don't know which printing I have.) Page 120, Section 4.5 "No gratuitous incompatibilities with C". It explains my original claim, and also why it couldn't be 100% compatible due to some other C++ desirables (eg better type safety). Partial quote:
"C++ doesn't aim at 100% compatibility with C because that would have compromised the aims of type safety and support for design. However, where these aims are not interfered with incompatibilities are avoided - even at the cost of inelegance. In most cases, C incompatibilities have been accepted only when a C rule left a gaping hole in the type system."