FIFOCache keeps a sequence of keys internally, limited in size to :threshold
Each element entered into FIFOCache causes the existing sequences of keys to be concat with [new-key]
It is trivial to cause a StackOverFlow error under these circumstance as described by Stuart here: http://stuartsierra.com/2015/04/26/clojure-donts-concat
I've recently experienced this while working with a FIFOCache as part of a Storm topology.
I'm happy to work on a patch if that would help.