Munster were bidding to become the first side ever to begin a Pro12 season with three maximum point victories and seemed to be course for just that when Stephen Archer’s 57th-minute try opened up a 10-point gap.
Prior to that, Keith Earls registered his first try of the season whilst the Munster forwards also ground out a penalty try.
However, the visitors suffered three yellow cards in the final half hour as things went from bad to worse at a venue which continues to cause them problems.
For Benetton, Ludovico Nitoglia was the hero with a brace of tries, while Matt Berquist landed 19 points with the boot to hand Treviso their first win of the season.
“We’ve just taken a massive step backwards,” admitted Penny afterwards.
“You can’t afford to lose four players, as we did over the course of 80 minutes, and hope to get away with it.”
Having been beaten out the gate at the same venue last season, Munster couldn’t have got a better start, Earls dotting down for a superb try after just seven minutes.
The Limerick man was making his first appearance since sustaining a shoulder injury in April but showed no signs of rustiness linking with Simon Zebo to sprint home. The try owed much to the clever interplay between Casey Laulala and James Downey.
Benetton upped the pace. With the going never easy in the forward exchanges, Niall Ronan compounded matters when he was sin-binned for throwing a punch.
Out-half Berquist knocked over two penalties to nudge Benetton ahead.
Munster regrouped and almost bagged another try when Earls again slipped the cover defence to dot down, only for the TMO to rule against him. Undeterred, the visitors hammered away at the Benetton line and forced a penalty try after a series of powerful scrum drives.
Ian Keatley converted, and with his next contribution found Earls with a cross-field kick, who was only stopped by an outstanding tackle by Luke McLean.
Munster coughed up a penalty just before the break, and went down the tunnel with a narrow 12-9 lead.
Into the second half, Munster stole a march seven minutes in when Archer barged his way over the line for try number three, converted by Keatley. Benetton hit back however with a try of their own, Nitoglia going over and Berquist converted to reduce their deficit to three points.
The ex-Leinster man then went one better with a long-range penalty to even up the scores midway through the half, before Nitoglia got in for his second try of the evening.
Berquist converted and added a penalty to leave Munster trailing 29-19, with Duncan Williams and Cathal Sheridan both now in the bin (soon followed by Stephen Archer). Munster threw everything at Benetton but left without a bonus point.
Scorers for Treviso: Tries: Nitoglia 2 Cons: Berquist 2 Pens: Berquist 5
Scorers for Munster: Tries: Earls, Pen Try, Archer Cons: Keatley 2
BENETTON: L McLean; C Loamanu, M Capagnaro, A Sgarbi, L Nitoglia (F Semenzato 77); M Berquist, E Gori; M Rizzo (I Fernandez Rouyet 65), E Ceccato (G Maistri 50), A De Marchi (P Di Santo 74); A Pavanello (capt), M Fuser (D Budd 59); S Favaro (V Bernabo 73), A Zanni, M Vosawai (R Barbieri 51).
MUNSTER: D Hurley (JJ Hanrahan 66); K Earls, C Laulala, J Downey (C Sheridan 62), S Zebo; I Keatley, D Williams (I Dineen 72); D Kilcoyne (J Cronin 70), M Sherry (Varley 62), S Archer (J Ryan 74); D Ryan, B Holland (D O’Callaghan 62); P O’Mahony (capt) (CJ Stander 55), N Ronan, J Coughlan
Referee: Leighton Hodges (WRU)