Visual Studio is missing/moving my breakpoints

I got this problem after Windows had inexplicably added 6 months to the current system date (and 1 hour to the time). I didn't correct this right away, so builds made before the correction looked more recent to Visual. This lead to running the wrong (older) build when debugging.

Since I couldn't find a "Clean Solution/Project" option, I had to manually delete all .pdb files in the solution. The problem was instantly fixed. I just hope there aren't any more files I might need to delete (I'm new to Visual and don't know much about how it works behind the scenes).

Also check to see if you had set the breakpoints in one mode (Debug) but are now in a different mode (like Release). This caused me some momentary angst.

It's because the debugger isn't able to break at that point. For example, the debugger can't break on auto-implemented properties, or at the header of a method; instead, it breaks at the first line of the method.