title: "The breakpoint will not currently be hit. The source code is different from the original version." What does this mean?
body: <p>When debugging in Visual Studio, sometimes I add a breakpoint but it's hollow and VS says "The breakpoint will not currently be hit. The source code is different from the original version." Obviously this prevents me from being able to debug.</p>
  
  <p>What on earth does the message mean? What original version? If I've just opened up the solution and not made any changes whatsoever to the code, how can there be an 'original version'?</p>