I've had a lot of problems with FATAL ERRORS lately within my grading projects. It seems like after a few days of editing and re-editing the grading file, the drawing comes to a point of critical mass where the internal errors start stacking up. If you run the "AUDIT" command, you'll see. But it doesn't seem like AUDIT ever repairs the errors it finds as it reports. And sometimes there comes a point where you can't even run AUDIT or even save the drawing, which renders the drawing session useless. I can't remember right now the exact error message in the dialog. I'll make sure I post a screen cap next time it happens. But I do know that when that dialog appears, you can do nothing except CTRL-ALT-DEL out and end the AutoCAD session with no save. Then restore your last previously saved file. Not fun.
I don't have any magic remedy for this here. The advice I received from Autodesk was to take the previously saved drawing, run "-PURGE", then run "AUDIT". Works for a little while until I get the previously mentioned dialog box again. It usually happens within a few minutes of editing the drawing, usually when editing a feature line or grading.
So what I've done is to redefine the "QSAVE" command to run "AUDIT" before the save. This seems to help cut down on errors created by editing feature lines, gradings, etc. I use this QSAVE often when editing grading files, especially before a "major" edit.
Here is the simple AutoLISP code that you may copy to a .lsp file, then load into your startup application suite "APPLOAD".
(command "_undefine" "_QSAVE")
(command "audit" "Y")