  Within the model-independent framework of $SU(3)\times SU(2)\times U(1)$
gauge-invariant dimension-six operators, we study flavor-changing $Wtq$
couplings ($q=d,s$) and related four-quark contact interactions involving the
top. We obtain bounds on those couplings from Tevatron and LHC data for
single-top production and branching fractions in top decays, as well as other
experimental results on flavor-changing neutral-current processes including
$B\to X_{q}\gamma$ and $Z\to b\bar{q}$ decays ($q=d,s$). We also update the
bounds on flavor-diagonal $Wtb$ couplings using the most recent measurement of
$W$-helicity fractions in top decays from top-pair production.
