Yup, this occurs often and it's unpredictable.  I believe there is an obscure memory leak in the NCE code and it’s very difficult to reproduce, which means it's almost impossible to fix (in modern operating systems, memory is “freed” by the actual operating system, not the software, so if there is a memory bug, then it becomes a function of your actual hardware, what programs you currently have running, and what you’re trying to do in OpticStudio).

Until a smart developer at Zemax can fix this issue, I found in most cases closing down OpticStudio and restarting it will prevent further issues.  If you still experience this bug after restarting OpticStudio, open a ticket directly with Zemax and (if possible) share you sanitized file because this might help Zemax finally fix this issue.

Thankyou Michael, I am surprised that more users haven’t experienced this behavior… I had sent a file that was exhibiting this behavior to Zemax Tech Support a couple years ago… but they said the could not reproduce the overwrite bug.

I can confirm that this happens to me as well, from time to time. I think there’s a correlation to after I have copied an object that other objects are changed to this object type. But as Michael said, it’s very difficult to reproduce.

/Hans

+1.  This is a very annoying and time-wasting bug.

+1 (and all of our team members). This has been there for a long time. Weirdly enough, I experienced it more frequently when I remote into other machines (workstations) with Remote Desktop… 

+1 here, restarting OpticStudio is the go-to solution.

I know, but you can’t really save before restarting, because that object type has changed. It’s highly likely that the new object type parameters do not match, defaulting the parameters back.

So, you have a tough choice:

• Save and accept that you loose info on that object parameters.
• Close it without saving and hope that you didn’t do too many changes in the meantime.

None of the two is nice...

I also experience this more frequently when using a remote windows machine through VMware Horizon Client. 

If you notice it when it’s happening you can click undo to repair the damage. Then save and restart.