  We discuss the abstract structure of sequential weak measurement (WM) of
general observables. In all orders, the sequential WM correlations without
post-selection yield the corresponding correlations of the Wigner function,
offering direct quantum tomography through the moments of the canonical
variables. Spin-half sequential measurements are, on the contrary, constrained
kinematically, they are equivalent with single WMs. In sequential WMs with
post-selection, a new anomaly occurs, different from the weak value anomaly of
single WMs. In particular, the spread of polarization $\hat\sigma$, as measured
in double WMs of $\hat\sigma$, will diverge for certain orthogonal pre- and
post-selected states.
