When the borrow check fails in a loop, the error message can be quite confusing:

enum Nat {
    S(~Nat),
    Z
}
fn test(x: &mut Nat) {
    let mut p = &mut *x;
    loop {
        match p {
            &Z => break,
            &S(~ref mut n) => p = &mut *n
        }
    }
}

The borrow checker informs me:

Test.rs|43 col 16 error| cannot borrow `*(*p)#0` as mutable more than once at a time
Test.rs|43 col 16 n| second borrow of `*(*p)#0` as mutable occurs here

This message is confusing, because the two occurrences have the same line number and column. In fact, the error message is right: the borrow check only fails because it is being performed in a loop. The error message should mention something like that to the effect.