I had some Rails code that broke when we went from MRI to JRuby, which I tracked down to this difference:

1.9.3-p448 :001 > Time.utc(2014, 2, 11, 24, 0)
=> 2014-02-12 00:00:00 UTC

2.1.0 :001 > Time.utc(2014, 2, 11, 24, 0)
=> 2014-02-12 00:00:00 UTC

jruby-1.7.10 :002 > Time.utc(2014, 2, 11, 24, 0)
ArgumentError: argument out of range.
from org/jruby/RubyTime.java:1114:in `utc'
...

The hour 24 is apparently ok according to the ISO standards, and the time formatting in iOS happily generates them, so we see a lot of them in our requests.