Wednesday’s winning run of four games was brought to a halt by Brentford, who emerged 2-1 winners at Hillsborough.
First half headers from John Egan and Harley Dean upset both the form book and the home fans, who faced a night of frustration, despite Fernando Forestieri netting a late consolation.
The Owls maintain their seven-point lead over Norwich in seventh place but will head into Saturday’s derby at Leeds disappointed that the advantage over the Canaries was not extended.
Despite a much-improved display in the second half, the damage was done in the first, when the best that Wednesday could point to was a controversial penalty appeal that went the way of the visitors.
Through an opening that Brentford edged, the Owls struggled to find their momentum, which picked up around the midway point.
Twice inside three minutes Almen Abdi made the Bees’ rearguard work to keep the Swiss international at bay.
First Abdi tested keeper Daniel Bentley from range with a dipping volley before blasting an angled drive goalwards stopped only by the midriff of Egan.
Then came the big talking point of the opening 45 as Forestieri appeared to be bundled over in the box by Dean but the referee waved away the Wednesday appeals.
It seemed harsh on the Owls – and moments later, on 36 minutes, Brentford went ahead.
Ryan Woods swung over a high set piece that fell nicely to Egan, who looped a header over Keiren Westwood and in.
Wednesday upped the tempo and responded with Jordan Rhodes grazing the crossbar from a Liam Palmer feed.
But Brentford had the final word before the interval as they doubled their lead almost on the whistle.
Josh McEachran aimed an accurate corner into the danger zone right on to the head of Dean, who powered his header home from six yards.
Still there was time for Dean to deny Rhodes with a fine block almost on the line and Wednesday went down the tunnel facing an uphill battle after the break.
They certainly came out of the traps apace, dominating possession and asking all the questions.
Time and again the Owls probed but chances were few – Abdi hit over the bar from distance, Forestieri narrowly missed the target and Ross Wallace dragged wide.
Substitute Callum McManaman caused problems down the left flank and twice went close to halving the deficit.
Forestieri forced two superb saves from Bentley in the closing stages as the one-way traffic continued and in the last five minutes Rhodes saw a glancing header cleared off the line and final substitute Steven Fletcher hit the post.
Still there was time for Bentley to save athletically from Wallace before Vincent Sasso stabbed wide from six yards.
At last the Owls got their reward deep into stoppage time when Forestieri curled a delightful effort into the top corner but it was not enough as Brentford held on to take the points.
Wednesday: Westwood, Palmer, Loovens (Fletcher 81), Sasso, Reach, Wallace, Hutchinson, Abdi (McManaman 66), Bannan (Winnall 46), Forestieri, Rhodes