You want to forever delete the email messages in Trash... You click on
Trash. But there, you find there are some messages you don't want to delete, not yet... So you think of filtering those you want to delete. The search tab already has
in:trash written in, so you go on filtering upon your heart's desire.

At this point, I would humbly expect to get a
Delete Forever button, instead of the
Delete one. After all, the button
was there right before I made the search. Oh well, I'll just click
Delete.

A ha, that
was a waste of time, just like I imagined - the action
Delete applied on a conversation that's already in the Trash, will unnecessarily move it to the Trash.
I
could select the needed messages, go and click on
Trash again, and
forever delete them from there, but that's a little counter-intuitive.
I just added my suggestion regarding this issue, at the
Gmail: Help Center. I don't feel like searching the Gmail dedicated Google Group(s) for this one, sorry.