A glaring error by goalkeeper Allan McGregor gifted Belgian side Lokeren a narrow advantage over Hull in their Europa League play-off first leg.
The 32-year-old passed straight to Hans Vanaken before the Lokeren midfielder rounded the former Rangers stopper and rolled the ball into the net.
McGregor had earlier kept out Nill De Pauw's fiercely struck 18-yard shot.
Hull forward Yannick Sagbo wasted a great chance from close range while David Meyler was denied late on.
|BBC Radio Humberside's David Burns|
|"What was Allan McGregor doing? That's a calamity. There was no danger there."|
It leaves the Tigers with it all to do in the second leg at the KC Stadium on 28 August if they are to make the group stage in their first season of European football.
Steve Bruce's side did come from behind to beat Slovakia's Trencin in the previous round.
But their lack of cutting edge in front of goal, on a day when the Tigers had a bid for Blackburn striker Jordan Rhodes rejected, will concern Bruce.
Former Monaco striker Sagbo scuffed a seventh-minute chance when he should have scored, while Sone Aluko and Robbie Brady also wasted opportunities to score an away goal.
The Tigers boss made nine changes to the side that started the 1-0 win at Queens Park Rangers on the opening day of the Premier League season, with 21-year-old defender Harry Maguire making his debut after signing from Sheffield United last month.
Hull were comfortable for long periods but McGregor's howler could yet prove costly.
There was no danger when he received the ball in the 58th minute. Vanaken made no mistake in scoring and left the goalkeeper holding his head in his hands.
Hull pushed for a late equaliser, and their 1,200-strong travelling support were denied when Davino Verhulst tipped Meyler's attempt over after good work by Aluko.
And their frustrating night was summed up when James Chester somehow managed to miss the target with a free header from Aluko's cross.
Hull City manager Steve Bruce:
"We've made a stupid mistake and it's cost us the match.
"Allan won us the match on Saturday and then he makes a mistake like that. That's the life of a goalkeeper for you.
"Up until that moment we were very comfortable. We were going well and all of a sudden the roof caved in."
- 19Dutra JuniorSubstituted forLeyeat 83'minutesBooked at 90mins
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- 6Ricciardi Oyama
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