  Jet substructure techniques such as subjet $p_T$-asymmetry, mass-drop, and
grooming have become powerful and widely used tools in experimental searches at
the LHC. While these tools provide much-desired handles to separate signal from
background, they can introduce unexpected mass scales into the analysis. These
scales may be misinterpreted as excesses if these are not correctly
incorporated into background modeling. As an example, we study the ATLAS
hadronic di-$W/Z$ resonance search. There, we find that the substructure
analysis -- in particular the combination of a subjet asymmetry cut with the
requirement on the number of tracks within a jet -- induces a mass scale where
the dominant partonic subprocess in the background changes from $pp \to g \!+\!
q/\bar q$ to $pp \to q\bar{q}$. In light of this scale, modeling the QCD
background using a simple smooth function with monotonically decreasing slope
appears insufficient.
