We experienced 3 bugs that other guilds have also experienced. We were unable to kill him due to these bugs.
1. Hakkar's mind control ability would permanently wipe aggro of the affected character and not return it to him. He became essentially untankable.
2. Blood siphon did not damage Hakkar despite the entire raid being affected by the Poisonous Blood debuff.
3. We experienced Hakkar randomly drop 20% health for no apparent reason.
We have a video but it is unavailable at the moment. I will update this post when/if I am able to.
Furthermore, I tried to report this on the bug tracker but I have no idea how to work that thing and gave up.