What goes wrong if you mutate an object after using it as a HashMap key or HashSet element?

The map places the entry in the bucket determined by the key's hash at insertion time, so mutating a field that participates in hashCode moves the object's logical bucket while it physically stays put.

After that, contains and get compute the new hash, look in the wrong bucket, and report the element missing even though it's still in the map, effectively leaking it. The rule is to use immutable keys or at least never mutate key fields while the object is in a hash structure; records make excellent keys precisely because they're immutable.