I'm unsure of the context behind this, but it's possible to have a tab open that is not accounted for by the tab view — a 'ghost tab'. It can be opened by typing its URL into the URL bar and selecting it from the 'tabs' section.
This is concerning, considering tabs that are unaccounted for can run in the background, and may even be 'escaping'.
Here's a visual representation of my concern.

As you can see, this readme (MythicApp/wine) is not present in the tab view.
It must be accessed through other means.
The expected functionality is that all tabs are available to be accessed via the tab view.
Version 0.99.137-beta (WebKit 623.1.8.0.0)
Tahoe (26)