Two goals in the final 30 seconds ensured Boro's friendly meeting with Spanish second division side AD Alcorcon ended honours even.
Jordan Rhodes appeared to have won the game for Boro in the final minute, but the Spanish scored the second equaliser of the evening straight from the re-start to make it 2-2 as the final whistle was blown.
It was an evening which saw Victor Valdes and Alvaro Negredo make their first appearances in Boro colours and the latter marked it with a first half goal which ensured an internal lead.
Valdes made one smart save early in the game, but it was Boro who made most of the running.
The first chance came after 10 minutes when a strong challenge from Negredo saw George Friend free down the left. The defender fired a low cross towards Albert Adomah before it was cut out and cleared
Friend continued to give Rafael Cardona a tough time down the left and the Alcorcon defence had to make a smart clearance after the Boro man lashed another dangerous ball across the face of goal on 15 minutes.
Jose Benito in the Alcorcon goal made himself big on 30 minutes to smother Adomah's effort after the midfielder was slid through in the box with a lovely disguised pass from Gaston Ramirez.
But the breakthrough came on 34 minutes when a Ramirez shot came off the post to debutant Negredo in the box. He moved the ball around two defenders to create space before finishing past Benito with his right foot.
Negredo almost doubled Boro's lead four minutes later after another terrific cross across the box from Friend. The striker aimed to lift the ball over Benito but the goalkeeper did well to tip it over.
Karanka made five changes at the break, with Tomas Mejias, Emilio Nsue, Cristhian Stuani, David Nugent and Jordan Rhodes all coming on for Valdes, Barragan, Adomah, Ramirez and Negredo.
A ball over the top proved to be Boro's first chance of the second half, as David Nugent brought it down but fired his shot over from inside the box.
But Alcorcon levelled the score 10 minutes into the second period when Bautista shot from 25 yards. His effort swerved and wrong-footed Tomas Mejias who got a hand to it but was unable to prevent the goal.
Boro applied some late pressure with a few minutes left when Adam Forshaw broke down the right to fire a cross into the area but Alcorcon did well to clear from a crowded box.
Boro's second and what looked like the winner came with a minute to spare after De Pena lifted a cross from the left on to the head of Rhodes who headed home for his fourth goal of pre-season.
But Alcorcon managed to salvage a late equaliser, though, as a lose ball n the box who smashed the ball past Mejias.
Boro: Valdes (Mejias 46), Barragan (Nsue 46), Morris (De Sart 71), Gibson (Baptiste 71), Friend (Reach 71), Clayton (Forshaw 71), De Roon (Fischer 71), Adomah (Stuani 46), Ramirez (Nugent 46), Downing (De Pena 59), Negredo (Rhodes 46).
AD Alcorcon: Dmitrovic, Benito, Cardona, Llorens, Vega, Pedreno, Candela, Rodriguez, Santiago, Teijero, Gutierrez. Subs: Razvan, Peralta, Lopez, Serna, Alonso, Vazquez, D Lopez, M Lopez, Tapia, Romagnogli.