Undertaker and Brock Lesnar.
 

What's all this beef about?

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WWE trying to get Brock to come back to WWE once his UFC contract expires

Supposedly just relates to negative comments they made about each other back in the past.

Although I've also heard Vince put Undertaker up to it, so who knows really.

Ah right, just found the Lesnar response video on YouTube. Pretty much says negative comments filtered through a mutual friend.

One of them didn't want the other to go over in a match, if I remember somewhat correctly.

That was at Unforgiven in 2002. A month later at Bad Blood, Undertaker put Brock over in the Hell In A Cell.

Didnt know it was possible to be a dick and a pussy at the same time, but Taker did it

I remember there was a rumour that the Undertaker's first feud after returning with the dead man gimmick at Wrestlemania 20 against Kane was to feud with Lesnar. I believe Undertaker was supposed to go over Lesnar and this may have influenced Lesnar's decision to leave WWE.

Which also explained his brief feuds with first Booker T, and then with the Dudley Boyz and Paul Heyman.

Rumor has it that Taker was having second thoughts about putting Lesnar over prior to their match at No Mercy 2002. Taker had to threaten Taker to do the right thing for the business or else he would kick his own ass. Lesnar ended up getting the win but that's when their relationship started turning sour.

Brock Lesnar has a tattoo of a sword on his chest. Cant take him seriously anymore. Meaning that Brock Lesnar is NOT serious business