It was planned. The storyline in the Vince/Ministry angle is that Taker started to believe in his own gimmick. Vince said it in a promo before WM15, "I think you started to believe in the gimmick, you started to believe that you're really "The Undertaker", Mark Calaway", something like that. So it wasn't the first time. That was a Russo style reality-based work/shoot angle.
I never knew thats the way they were going with that storyline, but the whole Mark Callaway believing in his own gimmick is brilliant. It's sad I don't remember and others don't, I find that absolutely interesting.
I do remember Shamrock calling Undertaker 'Mark' though, I knew that was his name and I also at the time thought it was a bit daft. Im sure I remember Undertaker falling from the ring or something and looked to have pulled his groin pretty bad in a match with Shamrock, which I think he took time off for. I'm sure it was probably already injured, but I can remember watching it again a couple of months later when he was out thinking I was really smart I could see when he hurt himself.