  To test the quality of a tunnel junction, one sometimes fits the
bias-dependent differential conductance to a theoretical model, such as
Simmons's formula. Recent experimental work by {\AA}kerman and collaborators,
however, has demonstrated that a good fit does not necessarily imply a good
junction. Modeling the electrical and thermal properties of a tunnel junction
containing a pinhole, we extract an effective barrier height and effective
barrier width even when as much as 88% of the current flows through the pinhole
short rather than tunneling. A good fit of differential conductance to a
tunneling form therefore cannot rule out pinhole defects in normal-metal or
magnetic tunnel junctions.
