500th tab in Safari ?

What happens to the 500th tab in Safari when you open another (501st tab) via an external link/source?

It seems the 500th tab has gone missing and is replaced by the new one. But it won't let me go 'back' to the 500th, and it has wiped over the 500th tab. The 'back' arrow is greyed out, so cannot go back to it.

How can I easily find what my 500th tab was?

Thank you.