Borussia Dortmund's defence regressed to their nadir of last season, going down 2-1 in a pre-season friendly to Bochum.
Thomas Tuchel had enjoyed three big wins from his first few games in charge of the Ruhr giants, but this defeat may give him food for thought.
Marco Terrazzino pounced after Henrikh Mkhitaryan had given the ball away in midfield to put Bochum ahead close to the half-hour, with Dortmund having dominated the opening stages.
An attacking diamond midfield was putting the top-flight side's defence under pressure at times.
And they buckled midway through the second half as Mats Hummels and Matthias Ginter made a total mess of attempting to clear in the box, both heading the ball back into danger before Tim Hoogland made them pay with a fierce effort.
Young Dortmund attacker Jeremy Dudziak showed composure that some of his senior colleagues lacked to score a consolation goal late on with a neat finish.
But it was not enough and Tuchel will hope to regain momentum against Luzern on Tuesday before a glamour tie with Juventus next weekend.
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