The Triangulum Galaxy is the warp problem distilled to one object. M33's outer HI disk bends and twists dramatically beyond the optical edge, one of the most pronounced warps known (Rogstad et al. 1976; Corbelli and Schneider 1997), in a galaxy that offers none of the standard excuses: it is relatively low mass, nearly bulgeless, dynamically fragile, and shows no signs of recent major interaction.
Each conventional warp mechanism fails M33 on its own terms. A tidal encounter strong enough to bend the outer disk this far should also have disturbed the delicate flocculent spiral structure, which survives intact; proper-motion constraints complicate a recent close passage with M31; misaligned cold accretion must be aimed just so; and a torquing triaxial dark halo must hold a tuned shape and orientation. The warp is also coherent and apparently long-lived, where excitation-based explanations predict winding and dissipation within a few rotation periods at these radii (Corbelli and Salucci 2000). An isolated, fragile galaxy carrying a strong, durable, organized warp is precisely the configuration the standard inventory of episodic mechanisms is worst at producing.
The standing makes M33 a benchmark test: any theory of warps must handle the case with no interactions to blame. Deep HI mapping and Gaia-era proper motions of the M31-M33 system keep tightening the constraints on every encounter-based story.