This happens to me, but only when I am running C3 in multiple tabs with different games - so I assume I'm confusing the program.
Let's say I'm trying to build some special menu that I used 3 games ago and I can't remember how I did it. So I open that game in another tab and start to copy and paste from one game to the other ...... then I see something stupid that I did in the second game and attempt to fix it and save ...... Typically, that's when I give C3 a coronary and I get that error.
Or if I have C3 open on 2 different laptops, that usually takes longer to get an error, but usually something goes crazy.
I use C3 in Chrome, not sure if the same thing happens in FF, but I assume it would.