As a noted MMA fan, it wasn't unusual to see The Undertaker (with wife Michelle McCool) sitting cage-side at UFC 121 in Anaheim, especially with ex-WWEer Brock Lesnar defending the Heavyweight belt in the main event. After Lesnar got his bell rung by Cain Velasquez, he just happened to be strolling away from The Octagon in a daze at the same time MMA journalist Ariel Helwani interviewed The Undertaker. The two heavyweights locked eyes briefly, with The Undertaker plainly asking Lesnar if he "wanted to do it."
The confrontation was staged on The Undertaker's part (watch Undertaker's eyes as he waits for Lesnar to approach). Reportedly, WWE was interested in using Lesnar in something of a Mike Tyson/Floyd Mayweather capacity at WrestleMania, using his real-sport celebrity to cross-pollinate fight fans in WWE's direction. The Undertaker's involvement was meant to insinuate the idea of a Lesnar match without outright tampering with a contracted performer.
However, Lesnar's exclusive UFC contract made it impossible to work out any sort of deal, and UFC face-man Dana White even publicly stated it was out of the question.