I got the WTF-message a couple of times too. If I remember correctly, it only appears during the rendering-process, no matter if there´s still a lot of free RAM. But max also crashes whenever I delete a a distribution object before deleting the CScatter and with displacement-maps if some normals are facing inwards, but then without the WTF-message.
But still, Corona is remarkable stable - and not just for an alpha-version. I can scatter millions and millions of objects and Corona only needs a few seconds to calculate them without cramming my ram. I can´t even tell how much I love this renderer, even with the sporadic crashes! Hopefully Alpha 6 is coming soon!