  We present DES14X2fna, a high-luminosity, fast-declining type IIb supernova
(SN IIb) at redshift $z=0.0453$, detected by the Dark Energy Survey (DES).
DES14X2fna is an unusual member of its class, with a light curve showing a
broad, luminous peak reaching $M_r\simeq-19.3$ mag 20 days after explosion.
This object does not show a linear decline tail in the light curve until
$\simeq$60 days after explosion, after which it declines very rapidly
(4.38$\pm$0.10 mag 100 d$^{-1}$ in $r$-band). By fitting semi-analytic models
to the photometry of DES14X2fna, we find that its light curve cannot be
explained by a standard $^{56}$Ni decay model as this is unable to fit the peak
and fast tail decline observed. Inclusion of either interaction with
surrounding circumstellar material or a rapidly-rotating neutron star
(magnetar) significantly increases the quality of the model fit. We also
investigate the possibility for an object similar to DES14X2fna to act as a
contaminant in photometric samples of SNe Ia for cosmology, finding that a
similar simulated object is misclassified by a recurrent neural network
(RNN)-based photometric classifier as a SN Ia in $\sim$1.1-2.4 per cent of
cases in DES, depending on the probability threshold used for a positive
classification.
