#1 Making him break kayfabe
It's no secret that every single Superstar that steps inside the squared circle is a mere mortal, but some characters have allowed fans to suspend their disbelief and see them as more. With that being said, The Undertaker was one of those special Superstars who was always in character and rarely broke kayfabe.
Unfortunately for The Undertaker, WWE pulled back the curtain on his Deadman persona during his WrestleMania feud with AJ Styles. Now, this doesn't mean the docuseries shouldn't have happened or that it revealed too much, it just would have been better served for a later time.
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In the end, WWE really shouldn't have pulled back the curtain during Undertaker's feud with AJ Styles and just allowed him to be The Phenom one more time. Sure, Styles poking fun at the real life Mark Callaway was an interesting angle and did court interest from fans, but it came at the expense of one of the greatest characters of all time.