#1 The Undertaker and Sting had a match outside WWE
The big WWE WrestleMania match which everybody wanted to see but never got was Sting vs. The Undertaker. Though these two legends didn't compete against each other at The Grandest Stage Of Them All, it doesn't mean that they never faced off in a ring.
The Undertaker or, more precisely, "Mean" Mark Callous, had a match with Sting years before "The Phenom" was born. The match took place in September 1990, shortly before Callous jumped ship to WWE and was handed the gimmick that would make him an icon.
The advertised match was Sting vs. Windham for the NWA title, but Mark Callous wrestled Sting instead. The match surprisingly was lackluster, and it wasn't televised. In the end, Callous lost the match. Shortly thereafter, Callous moved on to a new chapter in his career, and the rest is history.
It is extremely disappointing that fans never got to see a match between these two legends when they were in their prime.