  We attempt to constrain the initial helium abundance, the age and the
efficiency of the convective core overshooting of the binary system TZ
Fornacis. Our main aim is in pointing out the biases in the results due to not
accounting for some sources of uncertainty. We adopt the SCEPtER pipeline,
relying on stellar models computed with two stellar evolutionary codes (FRANEC
and MESA). We found multiple independent groups of solutions. The best one
suggests a system of age 1.10 $\pm$ 0.07 Gyr (primary star in the central
helium burning stage, secondary in the sub-giant branch), with a
helium-to-metal enrichment ratio of $\Delta Y/\Delta Z = 1$ and core
overshooting parameter $\beta = 0.15 \pm 0.01$ (FRANEC) and $f_{\rm ov}= 0.013
\pm 0.001 $ (MESA). The second class of solutions, characterised by a worse
goodness-of-fit, still suggest a primary star in the central helium-burning
stage but a secondary in the overall contraction phase, at the end of the main
sequence. The FRANEC grid provides an age of $1.16_{-0.02}^{+0.03}$ Gyr and
$\beta = 0.25_{-0.01}^{+0.005}$, while the MESA grid gives $1.23 \pm 0.03$ Gyr
and $f_{\rm ov} = 0.025 \pm 0.003$. We also show that very precise mass
determinations with uncertainty of a few thousandths of solar mass are required
to obtain reliable determinations of stellar parameters, as mass errors larger
than approximately 1% lead to estimates that are not only less precise but also
biased. Moreover, we show that a fit obtained with a grid of models computed at
a fixed $\Delta Y/\Delta Z$ can provide biased age and overshooting estimates.
The possibility of independent overshooting efficiencies for the two stars of
the system is also explored. This work confirms that to constrain the core
overshooting parameter by means of binary systems is a very difficult task that
requires an observational precision still rarely achieved and robust
statistical methods.
