What Causes This? (Unity Bug Reporter)

I have experienced this a few times now. I'm not sure why it happens. But sometimes when I change the scripts in my projects, the next time I try to open the project it doesn't open and shows me this instead.

(Luckily, I've learned to save backups after I make significant changes each time I edit a project so I haven't lost anything substantial). But I was wondering what type of code usually cause this to pop up when opening a project?

This only seems to happen when I change the code in my scripts. And it doesn't affect the game until after I close the project and try to open it again.