I take it you've never made a mistake programming before then? A relatively trivial example is passing by const reference [0]
While it's fairly clear in this example that you're getting a reference to a local variable, it can easily slip through if you pass something through a level or two of a function. Note there's no compiler warning for this. It also (probably) won't show up until you actually try and use St.m_x at some point, which may be in a very different place to where it was initialised. In most cases, if it shows up in an optimised build, you'll get next to 0 useful information from a debugger/stack trace.