I took time to follow the entire route of the two parts of the
condominium from the Luxembourg map site, and found that to the north
of Vianden, in the northern detached part of the condo territory,
there is an island.

Near (south of) Wasserbillig, in the southern sector of the condo,
there appears to be a part of the river that shifted or at low water
at least, doesn't fill the river bed allocated to it.... the border
varies a lot in that area from what is shown as "wet".

It's interesting to follow the border lines - especially in the two
places where there is a confluence of the three rivers that comprise
the condominium. You tri-pointers would have a ball. One river comes
in from the west in Luxembourg proper in the northern end and another,
the Mosel / Moselle comes in from being wholly in Germany in the east.
The town of Vianden (Lux), being on the east of the river, forces a
split of the condo into two parts, bercause as the river passses
Vianden N-S, it's wholly in Luxembourg; each end of the split has it's
DE-LUX-Condo tripoints, too.

LN