So, at work, we've got a pair of HP ProLiant servers, and like many modern machines, most of their slots are PCI-E and they've each got one legacy regular-PCI slot. Says so right on the sticker inside the case cover.
But when we look at these motherboards, it looks like each of their regular PCI slots are mounted backwards, whereas the PCI Express slots seem correct.
Here is a photo from the Wikipedia article about PCI Express, compared to a photo of one of our ProLiant boxes. Note the position of the divot, indicated by the red arrow in each photo.
Are these boxes truly backwards, or is this some oddity of the PCI standard we don't know about? Our PCI cards don't fit in this slot. (Well. They would fit if we took the bracket off the card. But we're afraid to try for fear of shorting things out if the slot is truly soldered into place backwards.)
(Wikipedia Photo First):