The Jaguares chose the worst possible time to end their seven-match winning streak when they succumbed to the Bulls.
Had the Jaguares won, they would have topped the South African conference ahead of Saturday's final round of league matches.
A big march could have been stolen on the Lions (41), who would have watched on nervously as a Jaguares triumph (38) could have left them with the worst-case scenario of finishing second and the dreaded proposition of an Australasian play-off
The Jaguares still have the benefit of the late kick-off against the Sharks in Durban where they'll have an idea of whether they should fly across the Atlantic or the Indian Ocean for their play-off. The Lions can't trust themselves to hold a lead and therefore can't be trusted with the conference leadership position.
John Mitchell's side needed the win to atone for last week's abomination in Singapore but there was plenty of room for improvement. However, they will be severely punished by the Lions if they display the first-quarter generosity that allowed the Jaguares to get a hefty lead.
Revenge, though, was well served after the 54-24 battering the Bulls received from the same team on May 19.The Jaguares showed they meant business by taking a 19-0 lead in 19 minutes but somehow they allowed the Bulls an undeserved half-time lead.
They didn't even have to work hard for their three tries, two of which were converted by flyhalf Nicolas Sanchez.
There was a serious dereliction of defensive duty as early as the third minute when Sanchez dotted down in the right corner.
The initial tackle was missed by lock Ruben van Heerden and the cover defence simply didn't have the gas and conviction to make the necessary try-saving tackle.
Jaguares captain Pablo Matera waltzed over for the second try two minutes later as the Bulls abdicated their tackling responsibilities. What was frightening was that the Bulls weren't falling off tackles, but seemingly couldn't keep up with the early tempo set by the Jaguares. It became even messier when Gonzalo Bertranou crossed the line in the 19th minute, a try that left the sparse Loftus crowd in shocked silence.
They did well to brave the freezing conditions to support their under-performing team but the shambolic opening quarter did their hard-earned rands no justice.
The Jaguares though are prone to over-elaboration and this worked into the Bulls' favour. They scored their first try in the 22nd minute through Johnny Kotze, who was the recipient of excellent work down the right- hand flank by Handre Pollard and Warrick Gelant. As the first half descended into a litany of errors, the Bulls built a good head of steam and scored a good 35th-minute try through scrumhalf Embrose Papier.The second-last pass from Gelant to Hanro Liebenberg looked suspiciously forward but the match officials saw nothing wrong with the offload.
Jesse Kriel then intercepted a pass in the Jaguares 22 and somehow the Bulls were in the lead while the Jaguares had done all the spadework.
The Bulls extended their lead to five points through a 43rd-minute penalty but Bertranou completed his brace two minutes later when the Bulls defence fell asleep.
The hosts got back into stride through a 52nd-minute Jamba Ulengo try created by a routine Pollard line break.
The Bulls finally gained ascendancy when Marco van Staden rumbled over from close range after the Bulls had made a dog's breakfast of a lineout.
Bulls (21) 43
Jaguares (19) 34
Bulls: Tries: Johnny Kotze, Embrose Papier, Jesse Kriel, Jamba Ulengo (2), Marco van Staden Conversions: Handre Pollard (5); Penalty: Pollard
Jaguares: Tries: Nicolas Sanchez, Pablo Matera (2), Gonzalo Bertranou (2); Conversions: Sanchez (3); Penalty: Emiliano Boffelli..