The Sagittarius Stream is the Milky Way's largest accretion event in progress: stars tidally stripped from the Sagittarius dwarf spheroidal wrap the Galaxy in a nearly polar stream spanning multiple full orbits, while the dwarf's repeated disk crossings appear dynamically linked to the Galactic warp, the vertical waves and corrugations in the outer disk, and the phase-space spiral Gaia revealed in the solar neighborhood (Ibata et al. 2001; Ruiz-Lara et al. 2020).
Fitting all of it at once defeats the standard machinery. The stream's full 3D track, its precession, and its puzzling bifurcation into parallel branches constrain the halo's shape; the disk's warp and waves constrain the perturbation history; and the halo configurations that match one set misalign or underpredict the other (Law and Majewski 2010; Deg and Widrow 2013). The notorious Law-Majewski solution requires a triaxial halo with its minor axis in the disk plane, an orientation dynamically unstable for disk galaxies, and later models trade that pathology for others. Twenty years of modeling the Galaxy's single most informative dynamical system have produced a sequence of mutually exclusive halos rather than a convergent measurement.
The standing is a benchmark crisis: the system every Milky Way potential model must pass keeps failing its modelers. Gaia proper motions along the stream and LSST depth on its faint wraps will tighten the data side decisively over this decade.