iframe

It does the nesting in your browser Kralizec?

Nesting? I'm not sure what you mean.

It was deliberate, not a bug.

I initially got the same idea as Sasaki. A frame can have a subframe but it won't work if it has the same URL as the original frame, apparently.
In the frame in my own post, I opened a subframe using Sasakis post, then opened this thread again and opened a sub-subframe using my own post again.

Nesting: frame inside a frame.

Edit: it doesn't show for me.