In Act 2 during Belial's first phase it is possible to bug him into doing no damage to you whatsoever, and given your timing is right even melee can hit him without a problem. It's really easy to replicate and shouldn't take you more than a couple tries to get the timing. His minions can still **** your shit but sometimes they will bug out and be unable to melee you as well.
There are two ways he can end up bugged, he can either stand at range forever and use his (easily avoidable) triple projectile, or he can path back and forth like a mad man forever. If you end up with the latter you're in luck because you can walk to him regardless of his position and he will continue pathing (most of the time! Sometimes if you get too close and try to kiss him he'll fix himself) for eternity.
Instructions:
Start the fight, as soon as belial jumps down to chase you, kite him to the southeastern half of the map area, then up against the inner railing run back towards your starting position, across to the railing behind it, and from there make a B-line to the corner in the screenshot below.
A screenshot I took (in normal, for ease of access) indicating the position I have always used.
If you don't understand, maybe a pictograph will help:
Order: 1,2,3, place Belial at star if you can, run to circle then B-line to the spot, hopefully Belial will be on or near circle.
Because I don't believe in reinventing the wheel, here is a video of what he looks like when he is truly bugged. (All credit for the video goes to the uploader)
It's really simple, though I'm not sure how you would survive phase 2 if you had to use this.