Charles: thanks for the comeback. I was building a cityscape. Lots of cubes, cylinders, and stuff. Lots of re-sizing, rotating, grouping, texturing, et al...but nothing arcane. My reply to bluetone is a pretty good description of the sequence of events. It seems that if you do more than one 'save as' the program kicks you into a loop. First, you get the 'An unknown error has occurred...". Clicking the 'OK' button is your only option. The dialog box goes away, and you can continue to work. But when closing time comes, "Save' is greyed out in the drop-down menu. 'Save As' is the only choice. If you close with an Alt_F4, Carrara asks if you want to save your changes. If you say 'yes', it then tells you again that 'An unknown error has occurred...' Back to Square One. Hope this helps. If you need more details, let me know. Am posting an image of the model.

And one more thing: of late, since I now only do CTL_S saves, the thermometer bar frequently does not flash on the screen. Since [when it does appear] it zips by very quickly, I assumed that I had not looked up soon enough and had simply missed it. But in the last three nights, when I went to close out, Carrara asked me if I wanted to save changes. I said 'yes' and every time I've re-opened, everything was there. But it seems as though even the CTL_S can be a 'sometime thing'