Background
Back in 3.2-3.3.x we used an avoidance capped block tank to tank the adds on the Anub HC encounter. This was because Nerubian Burrowers had stacking attack speed, low damage dealt and a percent damage modifier that was applied after block value. This meant you could stack avoidance to 101.6 and ~3k BV to effectively become immune to melee damage.
Proof
Video of it being tanked by a block tank in 2009 with combat log open showing no damage taken on blocks with all 4 adds on them and 10 stacks of expose weakness:
https://www.youtube.com/watch?v=-EWFz94HQWI
Example event from PTR:
https://imgur.com/a/mxWAPfr
Log the event is pulled from:
https://classic.warcraftlogs.com/reports/a32JVLcyjMAZ4vC7#fight=13&type=damage-taken&source=360&pins=0%24Separate%24%23244F4B%24auras-gained%24-1%240.0.0.Any%240.0.0.Any%24true%248610249.0.0.Paladin%24true%2467721&ability=1&view=events
Fix
Move the damage multiplier applied on hit to applying after damage is decremented by block.
Current:
(DamageTaken * ExposeWeaknessModifier) - BlockValue
Expected:
(DamageTaken - BlockValue) * ExposeWeakness Modifier