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+A value was used after it was mutably borrowed.
+
+Erroneous code example:
+
+```compile_fail,E0503
+fn main() {
+    let mut value = 3;
+    // Create a mutable borrow of `value`.
+    let borrow = &mut value;
+    let _sum = value + 1; // error: cannot use `value` because
+                          //        it was mutably borrowed
+    println!("{}", borrow);
+}
+```
+
+In this example, `value` is mutably borrowed by `borrow` and cannot be
+used to calculate `sum`. This is not possible because this would violate
+Rust's mutability rules.
+
+You can fix this error by finishing using the borrow before the next use of
+the value:
+
+```
+fn main() {
+    let mut value = 3;
+    let borrow = &mut value;
+    println!("{}", borrow);
+    // The block has ended and with it the borrow.
+    // You can now use `value` again.
+    let _sum = value + 1;
+}
+```
+
+Or by cloning `value` before borrowing it:
+
+```
+fn main() {
+    let mut value = 3;
+    // We clone `value`, creating a copy.
+    let value_cloned = value.clone();
+    // The mutable borrow is a reference to `value` and
+    // not to `value_cloned`...
+    let borrow = &mut value;
+    // ... which means we can still use `value_cloned`,
+    let _sum = value_cloned + 1;
+    // even though the borrow only ends here.
+    println!("{}", borrow);
+}
+```
+
+For more information on Rust's ownership system, take a look at the
+[References & Borrowing][references-and-borrowing] section of the Book.
+
+[references-and-borrowing]: https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html