Some of these have flanges and round heads, some of them have flanges and hexagonal heads.
The motherboard is attached by screwing little brass hexagonal standoffs into holes in the case, then resting the motherboard on the standoffs, and screwing it on.
USE THE SCREWS WITH FLANGES AND ROUND HEADS.
The screws with flanges and hexagonal heads are one-sixteenth of a wotsit too big, and prone to jamming in the standoffs. Mr Murphy will assure you that this will happen most in the standoff which is most inconveniently located on the motherboard.
Once jammed, the screw and standoff rotate as a unit, and there's not enough give in the motherboard for the standoff to rotate out of its hole in the case; if you could hold the standoff with pliers and unscrew the screw, this would be fine, but the standoff's underneath the middle of the motherboard at this stage.
The way that finally worked to unjam the standoff was to remove all the removable components and all the other screws from the motherboard, then pull very hard with a large pair of pliers on the head of the screw until the screw, standoff and motherboard flew out of the case as a unit. Detach screw from standoff by holding standoff in Big Pliers and turning screw with driver; discard screw and standoff, get new standoff and correct screw from bag of small screws, achieve glorious success.
This scraped some solder-mask off the motherboard tracks, but didn't actually break it. This time.
Of course, motherboards are not that expensive, so even if you break one from time to time it's cheaper to pay for the occasional whole motherboard than to pay the 15% overhead on all the components that your local shop will charge for assembly. But for a dual-core 64-bit high-performance processor to be stymied by a screw one sixteenth of a wotsit too wide is unavoidably irritating.