  Simulations of pre-supernova evolution suggest that some intense shell
burning can be so active that, in extreme cases, it can merge with the outer
shell, changing the initial conditions for the supernova explosion. However,
such violent activity in the interior of stars has been difficult to confirm
from observations of stars. Here we propose that the elemental composition of
O-rich ejecta in supernova remnants can be a tool to test for this kind of
intense shell burning activity in the final stages of progenitor evolution. As
an example, we discuss the origin of "Mg-rich" ejecta in the supernova remnant
N49B. A high Mg/Ne mass ratio $\gtrsim 1$ suggests that the Ne- or O-burning
shell has broken into or merged with the outer shell before the collapse. Such
Mg-rich (or Ne-poor) ejecta has been identified in some other supernova
remnants, supporting the idea that some destratification process, such as a
shell merger, does indeed occur in the interiors of some massive stars,
although they may not be the majority. Our results suggest that X-ray
observations of O-rich ejecta in core-collapse supernova remnants will be a
unique tool to probe the shell burning activity during the final stage of a
massive star's interior.
