The bug that caused Heartbleed was extremely obvious: read a u16 out of a packet, copy that many bytes of the source packet into the reply packet. If someone put that code in front of you in isolation you would spot it instantly (if you know C). The problem --- this is hugely the case with most memory safety bugs --- is that it's buried under a mountain of OpenSSL TLS protocol handling details. You have to keep resident in your brain what all the inputs to the function are, and follow them through the code.