user> (let [h (HashMap.)]
(.put h 1 2)
(let [p1 (first h)
p2 (first {1 2})]
(println (.hashCode p1) (.hashCode p2)
(= p1 p2) (= p2 p1)
(.equals p1 p2) (.equals p2 p1))))

3 994 false false true false

clojure.lang.MapEntry does not follow the contracts of hashCode or equals for java.util.Map$Entry. This, among other things, leads to the above non-reflexive behavior of .equals.

I realize this isn't a simple bug; it's a fundamental issue, in that it's impossible to consistently implement both java.util.List and java.util.Map$Entry, which clojure.lang.MapEntry claims to do. On balance I do find it very useful that Clojure map entries are equivalent to vectors, and probably wouldn't want to give that up. But, I think that at minimum this wart should be documented somewhere, since it could lead to some very strange and hard to catch bugs.