This is caused by the new shader instancing system in the demos; on some SM3.0 cards (two nVidia 6000 series tested so far), an OutOfVideoMemoryException will be thrown if simulations are changed too rapidly.
There are two workarounds; wait a few seconds between switching simulations, or open up the demos source.
The demos source contains all of the rendering systems, including the non-SM3.0 fallback, CPU instancing. This can be forced by creating the EntityDrawer in the switchSimulation(int sim) function within Demos.cs with useShaderInstancing = false. There is a performance hit, and the CPU instanced objects will be the same texture.
I'll be fixing this as soon as I get the chance.
Known Bug: Crash in demos on simulation switch
Re: Known Bug: Crash in demos on simulation switch
Alright, the new version should take care of it.