"Last October, I speculated on the best ways to go about designing and implementing a software backdoor. I suggested three characteristics of a good backdoor: low chance of discovery, high deniability if discovered, and minimal conspiracy to implement.
The critical iOS vulnerability that Apple patched last week is an excellent example. Look at the code. What caused the vulnerability is a single line of code: a second "goto fail;" statement. Since that statement isn't a conditional, it causes the whole procedure to terminate.
The flaw is subtle, and hard to spot while scanning the code. It's easy to imagine how this could have happened by error. And it would have been trivially easy for one person to add the vulnerability.
Was this done on purpose? I have no idea. But if I wanted to do something like this on purpose, this is exactly how I would do it.