The "X" formed where the administrative boundary between northern
Sudan and southern Egypt crosses the de-facto boundary forms a quad point.
Further west from this point, the border crosses lake Nasser - and the
border is undefined, but takes a dip northward into Egypt proper.
It's a weird place.

Lake Nasser (I think it was Nasser that it crossed - I don't have a
map in front of me), if I recall correctly, was formed by a dam - and
it's odd that maps show this little "dip" of the border into Egypt at
the lakeshores, as though the border changed from the straight line
that was there when the water showed up. Maybe it was "moved"
northward by the Sudanese when the lake grew close enough to Sudan so
they could dip into it without causing too much of a rucuss. It is
marked on some maps with a dotted line across the water, indicating
"in dispute" like the eastern end is.

Len Nadybal