Antonio Conte's champions-elect fail to find killer second goal against ten men and drop two vital points after horror moment from Italy international
Claudio Marchisio had given Antonio Conte's side an early advantage, but Buffon took a poor touch to allow Bertolacci to slide into an empty net. Coupled with AC Milan's win over Atalanta, the gap at the top is now back to just a single point.
Bianconeri boss Antonio Conte will be furious at his troops, who failed to take advantage of their obvious superiority and have handed rivals Milan some momentum in the title race.
An end-to-end opening suited the hosts who got off to a great start on eight minutes when Marchisio got on the end of a drifting Andrea Pirlo cross to nod past the flat-footed Massimiliano Benassi in the visitors' goal.
The visitors nearly found themselves two down on 26 minutes. A lovely passage of play saw the influential Marchisio dance through the middle and flight a pass to Arturo Vidal, but the Chilean fired wide of the upright from 12 yards out.
Marchisio then went close himself, Benassi turning the Italy international's bouncing 25-yard effort round the post.
The visiting Giallorossi started to knock a few passes around before the break, but entered the dressing room without mustering a shot on target.
Juventus emerged for the second period seemingly content to protect their one-goal cushion and their task was made easier on 54 minutes when Lecce striker Juan Cuadrado earned a second yellow card for a rash tackle on Uruguayan Martin Caceres.
Vidal almost exploited the extra man immediately, the Juve midfielder striding onto a Vucinic header only to hit a tame effort at Benassi.
Nearing the 70 minute mark, a comfortable Bianconeri nearly doubled their lead, Fabio Quagliarella hitting a delightful left foot volley just over from the edge of the area before testing a sprawling Benassi moments later with a low curling strike.
Juventus were struggling to put the visitors to the sword, however, and nearly paid the price when Luis Muriel almost latched onto a partially cleared free kick only to be denied by Buffon.
Muriel's half-chance stung the hosts into action and substitute Alessandro Matri crowned a fine run down the left flank with a firm angled shot - only to see Benassi palm the effort to safety.
Then Buffon's calamitous moment came with five minutes left on the clock. The keeper seemingly had acres of space, but took a shocking touch to allow Bertolacci to nip in and gleefully pass home into an empty net.
Marchisio went close for the hosts in a desperate finale, but the visitors hung on to gain an unlikely draw, which will go some way to soothing their relegation fears.
Juventus, meanwhile, will enter their final game of the season just one point ahead of AC Milan.