A disastrous stoppage-time own goal by Gerard Pique denied Barcelona the spoils in a 1-1 draw at Osasuna on Saturday as the champions saw their lead over Real Madrid cut to a single point.
Barca looked poised to win in Pamplona after Seydou Keita followed up his hat-trick against Real Zaragoza last weekend with a 73rd-minute strike but with seconds left former Manchester United man Pique diverted a cross into his own net for a Halloween howler.
"Gerard (Pique) tried to clear the danger and almost did but it ended up in the net," lamented Barcelona coach Pep Guardiola. "It is frustrating especially as it was so late in the game but these things can happen."
Barcelona now face a trip to Russia to take on Rubin Kazan in Wednesday's Champions League match and will need no warning about a potential slip-up, having lost 2-1 to the Russians at Camp Nou in their last European match.
On the domestic front unbeaten Barca stay top, though, 10-men Real closed in with a 2-0 home win over Getafe courtesy of a brace from Gonzalo Higuain.
Real bounced back from their mid-week King's Cup shambles against Alcorcon by defeating their derby rivals despite playing almost three quarters of the match with ten men.
Argentine Higuain was the hero, scoring twice in three minutes (53 and 56) as Real eased the pressure on coach Manuel Pellegrini following their 4-0 first-leg cup defeat to tiny Alcorcon on Tuesday.
"After a terrible week this was just the boost we needed," said Higuain.
"We now have Milan and Atletico so it was important to start with this win against Getafe."
Sevilla stayed in touch with the leading duo by winning 2-0 at Xerez.
Recently-capped Spanish international Alvaro Negredo and Brazil striker Luis Fabiano were on target as Sevilla closed to within four points of Barca.
Elsewhere, new Atletico Madrid coach Quique Sanchez Flores got off to a losing start on his league bow falling 1-0 at Athletic Bilbao on Saturday.
Highly-rated Javi Martinez, 21, scored on 19 minutes to keep Atletico fourth from bottom and dent confidence further ahead of Tuesday's vital Champions League home match against Chelsea.
It is a crunch week for Atletico who need to beat Chelsea to have any chance of qualifying for the knockout phase and then prepare for a derby against Real Madrid at the Vicente Calderon.
Barcelona captain Carles Puyol, 31, celebrated signing a new contract until 2013 this week and showed his versatility by deputising for the injured Dani Alves at right-back against Osasuna.
French striker Thierry Henry was back after a fortnight on the sidelines following a leg injury sustained on international duty with France and began on the substitutes bench.
Barca were brimming with confidence after last weekend's 6-1 drubbing of Real Zaragoza and almost scored in the first minute with Lionel Messi forcing a fine fingertip save.
Osasuna were far from over-awed though and Carlos Aranda should have scored in the 11th minute but shot wide of Victor Valdes's post.
The visitors almost took the lead following a training ground corner routine that culminated in a scuffed shot from Zlatan Ibrahimovic that appeared to be blocked by a hand but the referee waved away Barca's penalty appeals.
In the second half Barca put their foot on the pedal to peg back Osasuna and they got their reward when a clever move involving Lionel Messi ended with Puyol crossing for Keita to control and drill home his sixth goal of the season.
Osasuna were the last team to beat Barcelona in the league, winning 1-0 at Camp Nou on the penultimate weekend of last season, and they stole a draw at the death when Pique haplessly put the ball past Valdes for a painful own goal.
Barca now turn their attention to the Champions League, while Real must focus on a testing Champions League trip to AC Milan on Tuesday and then another derby against Atletico.