@Florian,@Jonas, of course you are both right, there is a race condition. I was looking for some inspiration for some sort of garbage collection for a (simple) script system I am going to create. In this case an "evil" user might cause such a problem by running two threads doing exactly this kind of thing. I read a bit further and I think I am going to use the Mark Phase algorithm for this.

Thank you very much, great work the compiler:)