Project Library file suddenly not found?

We experienced a strange error yesterday where all of a sudden our we received a overlay error in our production system. Upon investigating my team mate stated that there was a error where the system complained that it couldn’t find the “OEE class” in the Project Library? Nothing was changed in the system and no one had any Designer sessions opened at the time of the error. I had my team mate open the project up in the designer and just resave it. Miraculously that fixed the error and everything went back to normal. What I would like to know has another else experienced an issue like this? Unfortunately I was trying to look through the logs today to see if there was anything going on at the time the error happened, but the logs don’t go back that far (go fig).