  The lepton-number-violating decays of singly-charged Higgs bosons H^\pm are
investigated in the minimal type-(I+II) seesaw model with one SU(2)_L Higgs
triplet \Delta and one heavy Majorana neutrino N_1 at the TeV scale. We find
that the branching ratios {\cal B}(H^+ \to l^+_\alpha{\bar \nu}) (for \alpha =
e, \mu, \tau) depend not only on the mass and mixing parameters of three light
neutrinos \nu_i (for i = 1, 2, 3) but also on those of N_1. Assuming the mass
of N_1 to lie in the range of 200 GeV to 1 TeV, we figure out the generous
interference bands for the contributions of \nu_i and N_1 to {\cal B}(H^+ \to
l^+_\alpha{\bar \nu}). We illustrate some salient features of such interference
effects by considering three typical mass patterns of \nu_i, and show that the
relevant Majorana CP-violating phases can affect the magnitudes of {\cal B}(H^+
\to l^+_\alpha{\bar \nu}) in this parameter region.
