This is not really another heartbleed question. I don't want to go into massive detail on how the error happened, how heartbleed works, etc.
To summarize, the programmer (whether forgetting to, or just naivety) relied on the clients specification of how large the payload was. In this case it was a critical error because that alleged payload was returned to the client as confirmation/keep-alive/whatever.
Massive amounts of people are incorrectly calling this a buffer overflow attack, which it is not. The buffer was simply created to store an amount of data that was specified by the client.
My question is: Is there a well-defined term for this type of exploit, or even the type of coding overlook (I wouldn't call this an error outright, though some might and that's debatable)?