Since Solidity 0.4.3, the compiler now shows warnings when a block is not balanced. For example:

Error: Inline assembly block is not balanced. It leaves 1 item(s) on the stack.

I've tried finding documentation that gives into what this entails, and why it happens, but haven't found anything useful. What happens here? How do you fix this?

This internal getCode/codeAt function gives the above error (for example).