Like I'll be in Visual Studio or Word or Waterfox (a 64-bit performance focused build of Firefox) and suddenly focus is swapped for a split second and whatever word I'm typing gets cut off or a button I'm clicking isn't.
I found a very helpful post by Matt Gertz from some years back where he provides some VB.NET code for an application that can monitor processes that steal focus from it. I'm more of a C# guy myself, so I've run with Matt's core logic and created a 32-bit C# application in Visual Studio 2010 which provides similar functionality. It will also let you copy the log info to the clipboard to paste wherever for further examination.
I call it FocusMonitor, and you can download the source code.