Using 2D numerical hydrodynamical simulations of type Ia supernova remnants (SNR Ia), PhD student Danny 
Tsebrenko (Portman) and I show in a new paper that iron clumps few times denser than the rest of the SN ejecta might form protrusions in an otherwise spherical SNR. Such protrusions exist in some SNR Ia, e.g., SNR 1885 and Tycho. Iron clumps are expected to form in the deflagration to detonation explosion model of white dwarfs. For a possible alternative explanation, see news item from November 2014.
See news item in New Scientist.