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.
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'?
I suffered from this recently, and in my case I traced the problem back to something I was doing when testing: changing the system time. I'm not suggesting this is the case for everyone, but thought I'd mention it since it hasn't been mentioned already. It appears if you start moving the clock around between debug builds then it can get very confused about what order various files have been created it - I can only assume it is using file modified dates to determine if the source code is valid or not, and which binaries it needs to recompile.
It is also an option to re-save web.config to bump its modification time.