In Check.java, it attempts to refer to the following resource.

"unchecked.clash.with" (it means compiler.misc.unchecked.clash.with)

--------------------------------------------------------------------------
    static Object uncheckedOverride(MethodSymbol m, MethodSymbol other) {
    String key =
        ((other.owner.flags() & INTERFACE) == 0) ? "unchecked.override" :
        ((m.owner.flags() & INTERFACE) == 0) ? "unchecked.implement" :
        "unchecked.clash.with";
    return new Diagnostic(key, m, m.location(), other, other.location());
    }
--------------------------------------------------------------------------

In compiler.properties, it's being defined as follows.

compiler.misc.unchecked.clashes.with= {0} in {1} overrides {2} in {3}

It seems to be a misspelling of "clashes".