West Ham endured a chastening 2-1 defeat by Slovenian minnows NK Domzale in their Europa League third-round first-leg qualifier at Stozice Stadium.
Even with England striker Andy Carroll at the spearhead of a strong line-up, the Hammers came off second best against a club who were making their first appearance in Europe.
Slavin Bilic, who watched from the stands as he served the second game of his two-match touchline ban, saw Mark Noble pounce from the spot while the superb Matic Crnic scored both of Domzale's goals.
The rivals meet again in next Thursday's second leg at the Olympic Stadium where West Ham, who only avoided a 3-1 final scoreline when Adrian made a brilliant late double save, will demand a substantial improvement.
Bilic gave debuts to Havard Nordtveit and Sofiane Feghouli, but any preseason optimism evaporated when they fell behind inside the opening 11 minutes.
Nordtveit handled the ball in the area when a shot appeared to strike the underside of an outstretched arm and Crnic rifled home a sharp penalty that gave Adrian little chance.
Domzale's lead did not last long, however, as Axel Maraval clattered into Winston Reid from behind, sending the defender sprawling, and Noble was on target from the spot.
Chances fell at both ends with Crnic missing the best of them when he nodded wide a superb cross from point-blank range.
He made amends four minutes into the second half when Michail Antonio was caught out of position, enabling Crinic to charge into the area and slot a low shot past Adrian without a single tackle being attempted.
It was awful defending from West Ham, who exited the Europa League at the same stage last season, and they were unable to stamp their authority on the second half with the reflexes of Adriano saving their blushes at the end.