I've run into a rare undocumented feature that I can reproduce.

1 - Open a forum

2 - Click on a thread title

3 - Before the thread opens, move your mouse pointer over one of the menu items on the dark blue menu (Home - Forums - Members - Extras - Store - Help)

4 - If the black sub menu list pops up for even a split second, you get the white screen with just the FT logos and the thread does not open.

I know, don't do that!