Replace Mesh is unstable and crashes blender on occasion by clicking game Start.
File->New should ask if you want to save your current scene.
EDIT : today by clicking undo a couple of times Currupted a couple of the .blends script files.
Win7 x64. blender x64.
Yes… If You have not saved undo clears all edits in the text editor - It have hit me to… but You learn 
Nah this was a full on glitch, A undo’d a few lighting operations , then the scene glitched by not lighting all the objects. i decided to save and restart blender and then half of my scripts were garbled. very odd.
Have not seen that… scary.