With Mumbai chasing 163 to win, Mumbai opener Ryan Rickelton was struggling to score off Indian spinner Ansari when the batter holed out to cover, where Australian star Pat Cummins took a catch.
But the third umpire soon got involved and Rickelton was called back as replays showed Klaasen's gloves were marginally in front of the stumps when the ball was hit.
The laws of the game state: "The wicketkeeper shall remain wholly behind the wicket at the striker's end from the moment the ball comes into play until a ball delivered by the bowler touches the bat or person of the striker, or passes the wicket at the striker's end, or the striker attempts a run."
Ansari, who is playing in his debut IPL season, could do nothing more than give a wry smile as Rickelton received a second life and eventually scored 31.
He eventually finished with figures of 0-35 as Mumbai went on to win by four wickets, despite the earlier efforts of Australian opener Travis Head (28) and Abhishek Sharma 40 for the Sunrisers.
Ansari's no ball came a day after Mitchell Starc was also guilty of a rare back-foot no ball during the Super Over between Delhi Capitals and Rajasthan Royals.
The Australian bowler's back foot had touched the return crease, leading to the third umpire signalling a no ball.
The law states that the foot must land within the return crease and not touch it.Â
