I remember this shit from live. It happens if you begin your cast too soon after being hit. Melee push isn't completely instant, it takes a very brief moment of actual movement rather than warping you from one /loc to another. If you begin your cast during this .3 seconds of movement, it can get interrupted. This becomes almost completely unnoticeable at later levels when channeling skill is high, because the tiny amount of distance you move while casting during the last third of the pushback movement isn't enough to make a significant impact on your channeling check unless you're like level 4.
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