  3C 286 is a well-known calibrator source in radio astronomy. It is also one
of very few compact steep-spectrum sources (CSS) detected in $\gamma$-rays.
Here, we perform a detailed spectroscopic and variability analysis and present
the first quasi-simultaneous optical to X-ray spectral energy distribution in
order to reveal physical mechanisms which dominate its emission at different
wavelengths, and arrive at a reliable optical source classification. The first
main result of our study reveals several pitfalls when applying simple broad-
or narrow-line Seyfert 1 (BLS1 or NLS1) classification criteria which only look
at the [OIII]-H$\beta$ complex. [OIII] and H$\beta$ can be dominated by the
same outflow components, in which case FWHM(H$\beta$) is no reliable
classification criterion, and extinction by intrinsic or intervening material
can make the highest-velocity H$\beta$ component undetectable. After careful
combination of all information from UV-optical spectra along with
multi-wavelength data, we confirm that 3C 286 can be classified as NLS1 galaxy,
with line properties and SMBH mass (of order 10$^{8}$ M$_{\odot}$ and accreting
near the Eddington limit) close to the BLS1 regime, making it an important
borderline object. The quasi-simultaneous SED taken with $swift$ shows a sharp
rise in the optical-UV, implying the presence of a strong accretion-disk
component with EUV excess, consistent with emission-line diagnostics. Finally,
we report the discovery of X-ray variability of 3C 286, plausibly dominated by
jet emission, and variable by at least a factor $\sim4$. This result suggests
to exercise caution when using 3C 286 as radio calibrator in high-resolution
radio VLBI observations.
