There is no way they play test effectively. That's impossible with so many bugs. Unless they overlook them on purpose. It's possible they know about the bugs and do nothing about them. Nothing else makes sense.
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Back in the d1 days, they knowingly sent out an update that broke the health regen perks on everything in the game, when they were trying to Nerf the hungering blade perk on blade dancers. They pushed out a twab stating they knew it was broken but they pushed it anyway to see how it would play, and just kept it
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I remember the outrage on that, Red Death, Warlock regen went kaput. People were passed when Bungie said "we knew our patch would do this", than they 'fixed' it and wanted to see how the community liked the limited regent we got back.