This seems to be either only happening at resolution 320x200 or most prominently happening.
Use any barge in BG, Ship, carpet or carriage, and use the arrow keys or the numpad for movement. When you move to the west or east, the barge will not move in a straight line but shift to the south on every next tile.
This might be because the centering onto Avatar in barge.cc is not happening for each turning case.
tracked in our new issue tracker https://github.com/exult/exult/issues/489