I have had some errors of this sort. I find that it is best not to save to anything but bmp or psd because errors can occur. During the time that I used Strata I found that there were far more errors that occured, moving to Carrara in terms of bugginess was a dream becuase the problems are relatively few. My main problems is: Importing an OBJ created with wings as a vertex primative, subdivde smooting it then rendering with GI and indirect light can cause a render time error that freezes the program. I found that importing the object, Saving the scene and restarting the program seems to eliminate the problem.