due to the curvature of the Earth a really long building can't have walls that are both parallel and level.
Aparently, I've conflated Level with Plumb, the walls cannot be parallel and plumb. Due to the curve, the center of gravity for the walls would require them to angle in slightly together or not be plumb.
As a pediatric pedantic but fun point, in surveying, level lines are defined to follow the Earth's curvature. This means they are technically not straight. Horizonal lines, which are perpendicular to plumb lines, are often used as approximations for level lines.
Generally it's safe to use them interchangeably, but sometimes the distinction is important.