The desktop CLR JIT (at least x86) does dead-code elimination, even in debuggable code. This is obviously perfectly safe, transparent, (and good), from a program-execution perspective. But it can be a little surprising under the debugger because you can't set-next-statement (setip) to eliminated code.

In other words, if you have this C# code:
    if (false) {
        Console.WriteLine("Boo!"); // <-- can't setip here

You couldn't Set-next-statement to the "Boo!" line.  Normally this is fine, but every now and then it gets me. I think code-generators are more likely to produce code like that than a real person.

However, the JIT will not remove side-effect free expressions in debuggable code. For example,
    void Foo() {
        int x = 5;
        x ++;

The JIT will not remove either of those lines, even though 'x' is unused and so they don't actually do anything for program behavior. This means you can still set and hit breakpoints on those lines, which is what you'd expect from debuggable (non-optimized) code.

In optimized-code, anything's fair game and a function like Foo() would very likely be optimized out of existence.