1. If you have a merge take place in two lines right next to each other, say one variable is declared in one and a function processes that variable in another, if those two are responsible together for exiting a loop, then you're going to be solving the Halting Problem to show there are no bugs.
2. If you have one function "f(x) = a + bx + cx^2 +..." and another function that iterates through integers until a solution is found in integers, then if the internals of either is changed (either the constants in f, or the search strategy), then you're going to have to be solving "Hilberts 10th Problem" to understand that there are no bugs introduced, thought this was interesting as the changes are "further" apart.
At least for the general case, this should be a problem for all Turing Complete languages. It might be interesting to think about finding sections of code that are "Total", and perhaps put certain guarantees on that, but there still might be weird ways that two lines of code can play with each other beyond that.
I hope this isn't overly pedantic, or that I'm not completely off base.