  The singly charged $SU(2)_L$ singlet scalar, with its necessarily flavour
violating couplings to leptons, lends itself particularly well for an
explanation of the Cabibbo Angle Anomaly and of hints for lepton flavour
universality violation in $\tau \to \mu\bar \nu\nu$. In a setup addressing both
anomalies, we predict loop-induced effects in $\tau\to e\gamma$ and in $\tau
\to e\mu\mu$. A recast of ATLAS selectron and smuon searches allows us to
derive a coupling-independent lower limit of $\approx 200$ GeV on the mass of
the singly charged singlet scalar. At a future $e^+e^-$ collider, dark matter
mono-photon searches could provide a complementary set of bounds.
