Carpet in commit fc35c561a049d905763de37d60690fc15473d65d "CarpetLib: Some harmless code cleanup" (sic) replaced some assert(0) by CCTK_BUILTIN_UNREACHABLE().
One of those (in the switch in transfer_p_r in line 730 of data.cc) was just found by Zach Etienne to cause segfaults if one does not properly add a new interpolation operator.
The difference is that CCTK_BUILTIN_UNREACHABLE() tells the compiler that a given line of code will never be reached thus the compiler can optimize it away. assert(0) on the other hand makes no such statement, it just aborts if it is reached. This error is compiler dependent since CCTK_BUILTIN_UNREACHABLE() expands to CCTK_Abort(0,1) unless the compiler supports __builtin_unreachable.