Maybe talking about undefined behavior wasn't the best approach I could take :) But if you stick to simple machine languages then it shouldn't be a big problem. Something that is closer to Knuth's MIX than to x86 won't have much undefined behavior.

The thing that I was thinking about was that many operations that are defined in assembly language, such as integer overflow are undefined in C (and this undefined behavior is often abused by optimizing compilers) and C also abstracts over a lot of control flow conventions.