The most common AFAIK would be when using iterators. An iterator borrows (or mutably borrows, or takes ownership of, depending on how it's called; iter() vs ite

The most common AFAIK would be when using iterators. An iterator borrows (or mutably borrows, or takes ownership of, depending on how it's called; iter() vs iter_mut() vs into_iter()) the original collection, so the original collection can't be modified while being iterated. This means no "ConcurrentModificationException" or similar (or worse, silent corruption) can happen.