Diablo 4's Cow Level quest has a visual bug that could waste hours
The missing purple effect can make a completed quest step look broken, but there is a workaround.
The Secret Cow level quest is probably one of the longest and, at times, most annoying quests you will ever have to go through in Diablo 4. And even if we did get a few ways to shortcut it and skip at least the 3x666 cow farming, the next steps are not really easier or faster to complete.
And this gets even worse when you are running into a bug that makes it look like things are not working. So you stay there, trying to corrupt items over and over again, and it turns out they were already corrupted. Congratulations, you just lost a few hours for nothing...
If you are currently advancing your Cow Quest, you are probably trying to get a 
Rusted Bardiche. During that quest, there is a moment where you have to fight Cordycepic Zombies and drop specific items on mushrooms to corrupt them. The mushrooms are supposed to release some sort of purple smoke to indicate that the item is now corrupted.
However, in some cases, this smoke just never appears. You can log out, log back in, take another character, nothing works... the mushrooms are not doing their job.
So what exactly is the problem? Well, it turns out it is just a visual bug. The purple smoke is absent, but your item is already corrupted. So if you are playing with mushrooms and there is no purple smoke, simply continue with the next step of the quest, because chances are that your items are already corrupted and you can move on.
There is a small risk that the mushrooms are indeed not corrupting your items and you will be stuck, but usually you can just go to Kurast for the next step.
This definitely adds another challenge to this already challenging Secret Cow Level quest, but at least now you do not have to spend 2h trying to figure out why mushrooms are not releasing their purple smoke.