One that often gets people is this:

    struct foo {
        int bar;
    }

    void myfunc(struct foo *p) {
        int *t = &p->bar;
        if(p == NULL) return;
        *t = 5;
    }

Is this undefined? Probably. Absent compiler optimizations, it won't cause a crash on any system you're ever likely to encounter because it's not actually dereferencing the pointer, but some versions of gcc have caused security issues in Linux by optimising away NULL checks like this.