  A particular Riemannian metric which originally has been obtained for a
well-known coordinate system in the Euclidean 3-space, is shown to specify, in
fact, a manifold with boundary. There are two ways to make the manifold
complete. One is to identify two halves of the boundary that turns the manifold
into Euclidean 3-space as it was done originally. Another is to identify
boundaries of two copies of this manifold, that yields a complete manifold
which consists of two copies of Euclidean 3-space connected through a round
disk. In general relativity this kind of connection is called `loop-based
wormhole'. The straightforward calculation of curvature from the metric
specified yields an erroneous result, due which the curvature is zero, that is
impossible because a manifold with this structure cannot be flat. This paradox
is resolved in full correspondence with the generally-accepted definitions.
