The Iron Sheik has joined the grassy knoll lunch club ... claiming The Undertaker's loss at WrestleMania 30 was an off-the-cuff conspiracy triggered by an injury during the match.

It was a shocking loss ... breaking a record 21 wins in a row.

Undertaker suffered a serious concussion during his match against Brock Lesnar. A fringe cell -- including Iron Sheik -- believes Undertaker was supposed to win but after the injury there was a mad scramble to change the plan because Undertaker wasn't physically able to seal the deal.