West Ham endured a chastening 2-1 defeat to minnows NK Domzale in the first leg of their Europa League third-round qualifier in Slovania on Thursday.
Even with England striker Andy Carroll at the spearhead of a strong lineup, the Premier League side came off second-best against a club making their first appearance in Europe.
Havard Nordtveit handled the ball 11 minutes into his Hammers debut and Matic Crnic scored the resulting penalty to put the hosts ahead.
West Ham were level seven minutes later when Winston Reid drew a penalty and Mark Noble converted.
But Crinic scored his second goal four minutes into the second half after Michail Antonio was caught out of position, forcing West Ham to try to overturn a deficit in their first game at the Olympic Stadium next week.
In other third-round qualifying games on Thursday, Bundesliga side Hertha Berlin won their home leg over Danish club Brondby 1-0.
Vedad Ibisevic scored the only goal in the 28th minute with his back to goal in a fine overhead kick.
A Domenico Berardi penalty allowed Italy's Sassuolo to take an away-goals advantage over Lucerne after a 1-1 draw.
Marco Schneuwly opened the scoring for the Swiss side before Berardi equalised ahead of half-time. Markus Neumayr had a chance to win for Lucerne but his penalty was off the mark.
Ligue 1 club Lille were held to a 1-1 draw at home by Qabala.
Vitaliy Vernydub provided the Azerbaijani side with the lead on 13 minutes, though Lille were able to strike back through Ryan Mendes just after half-time.
Saint-Etienne's home leg against AEK Athens finished goalless.
Jonny Hayes grabbed a deserved late equaliser for Aberdeen in a 1-1 draw with Maribor.
The Dons missed a glut of chances in their home leg before being stunned by an 83rd-minute breakaway goal from Milivoje Novakovic.
However, Hayes slotted home from 10 yards four minutes later after good hold-up play from Adam Rooney to secure the draw.
Slovenia's Maribor have been the nemesis of Scottish football in recent years, knocking out Rangers, Celtic and Hibernian without losing a single leg.
Irish side Cork City stayed in contention to advance through a third round of qualifying after a 1-0 loss to Genk in Belgium.
Genk opened the scoring on the half-hour when Leon Bailey fired home following good build-up play down the left.