Why does F10 (step over) in Visual Studio not work?

I'm debugging a (web) project in Visual Studio 2008. I'm hitting a breakpoint. F10 continues to the next line, as expected, but the next F10 just stops debugging and the code continues without any more debugging (like pressing F5). Why is this happening?

• I have tried 'clean solution'.
• Other breakpoints sometimes(!) skipped, even in the same method

Any clues?

It is probably because you do not have the update kb 957912 for Visual Studio 2008. That fixed the same issue on my machine.