_str field in Keyword and Symbol classes lazily caches result of toString(). Because this field is not transient, serializing (using Java serialization) any keyword before and after calling toString() for the first time yields different results:
Uncomment (println :k) and suddenly s1 and s2 are no longer equal.
This issue came up when I was trying to use keywords as key in [Hazelcast](https://github.com/hazelcast/hazelcast) map. Hazelcast uses serialized keys in various scenarios, thus if I first put something to map under key :k and then print :k, I can no longer find such key.