A variable was borrowed as mutable more than once.

Erroneous code example:

let mut i = 0;
let mut x = &mut i;
let mut a = &mut i;
x;
// error: cannot borrow `i` as mutable more than once at a time

Please note that in rust, you can either have many immutable references, or one mutable reference. Take a look at https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html for more information. Example:

let mut i = 0;
let mut x = &mut i; // ok!

// or:
let mut i = 0;
let a = &i; // ok!
let b = &i; // still ok!
let c = &i; // still ok!
b;
a;