What is this..?

Solution
That's a grounding tab. Pull the motherboard out, bend that a bit so that it will stay inside instead of sticking out over the network jack, and re-seat the motherboard, and try to make sure that tab (and others like it for the other connectors) are on top of the connectors instead of in front of them. Or just bend it outward and let it break off, it's not critical.
That's a grounding tab. Pull the motherboard out, bend that a bit so that it will stay inside instead of sticking out over the network jack, and re-seat the motherboard, and try to make sure that tab (and others like it for the other connectors) are on top of the connectors instead of in front of them. Or just bend it outward and let it break off, it's not critical.
 
Solution




^ Please notice what I said before.



How will removing it mess up the alignment of the IO panel? I don't follow your logic there.
 
You built your own PC, what's 10 more minutes to do it right?

You could Try to bend it back and forth enough to get the metal to fatigue and break. But, if it breaks in the wrong place, and you can't plug in your ethernet cable, your screwed. Then you have to take the system apart and file down the metal. Good luck filing that thin piece thou.....