I have several macros set up on my DK to use Rune Strike when it procs. Since the start of the Cata Classic Pre Expac patch, my Macros are broken and try to use Rune Strike even when its not ready.
And unfortunately for you it’s not a bug. According to wowpedia this was intentionally changed in the first patch of Cata. No longer “on next swing” and incurs GCD