  We have identified three K2 transiting star-planet systems, K2-51 (EPIC
202900527), K2-67 (EPIC 206155547), and K2-76 (EPIC 206432863), as stellar
binaries with low-mass stellar secondaries. The three systems were
statistically validated as transiting planets, and through measuring their
orbits by radial velocity monitoring we have derived the companion masses to be
$0.1459^{+0.0029}_{-0.0032}$ $M_{Sun}$ (EPIC 202900527 B),
$0.1612^{+0.0072}_{-0.0067}$ $M_{Sun}$ (EPIC 206155547 B), and $0.0942 \pm
0.0019$ $M_{Sun}$ (EPIC 206432863 B). Therefore they are not planets but small
stars, part of the small sample of low-mass stars with measured radius and
mass. The three systems are at an orbital period range of $12-24$ days, and the
secondaries have a radius within $0.9-1.9$ $R_J$, not inconsistent with the
properties of warm Jupiter planets. These systems illustrate some of the
existing challenges in the statistical validation approach. We point out a few
possible origins for the initial misclassification of these objects, including
poor characterization of the host star, the difficulty in detecting a secondary
eclipse in systems on an eccentric orbit, and the difficulty in distinguishing
between the smallest stars and gas giant planets as the two populations have
indistinguishable radius distributions. Our work emphasizes the need for
obtaining medium-precision radial velocity measurements to distinguish between
companions that are small stars, brown dwarfs, and gas giant planets.
