Lukasz Fabianski's costly blunders left Arsenal trailing after the first leg of their Champions League last-16 clash against Porto at Estadio do Dragao.
The Polish goalkeeper, deputising for the injured Manuel Almunia, was caught out of position to gift Porto an early lead, before then picking up a back-pass which led to another goal at the start of the second half.
However, Sol Campbell's header which had levelled at 1-1 could yet prove to be crucial when the teams return to the Emirates Stadium in a fortnight's time.
Wenger had warned Porto were no European novices, the Portuguese side having consistently qualified for the elite club competition they won under Jose Mourinho in 2004. They were beaten by Manchester United in last season's quarter-final.
Porto, who lost to Chelsea in the group stages, almost got off to a dream start.
Hulk, serving a domestic ban following an altercation with a steward after a match against Benfica, cut in from the left before feeding Ruben Micael whose effort was blocked by Thomas Vermaelen.
Brazil striker Hulk then fired wide after another quick passing move, this time down the right channel.
At the other end, Arsenal captain Cesc Fabregas almost got clear, but Porto skipper Bruno Alves made a saving tackle in the six-yard box.
A terrible mistake from Fabianski gifted Porto the lead after 11 minutes.
The goalkeeper looked anything but comfortable in the recent FA Cup defeat at Stoke, and got his angles all wrong this time as a low right-wing cross from Silvestre Varela caught him too far forwards, deflected off his diving body and rolled over the line.
It was an almighty setback for the injury-hit Gunners, who had recovered well after a shaky start.
Arsenal, though, soon went on the offensive, as Nicklas Bendtner's near-post cross was cut out.
The Dane's 20-yard effort was then deflected behind for a corner, from which Arsenal drew level in the 16th minute.
Vermaelen flicked Samir Nasri's delivery from the left over to the far post, where Tomas Rosicky nodded it back into the six-yard box and veteran Campbell powered a header into the roof of the net for his first Arsenal goal since the 2006 Champions League final.
Fabianski went some way to redeeming himself with a flying save to keep out a 25-yard angled drive from Micael on 29 minutes.
Diaby went into the book after 32 minutes after chopping down Varela as the Porto winger looked to break down the right. Alves then swiftly followed after kicking the ball away as tempers started to boil over.
After 36 minutes, Bendtner's backward header from a deep free-kick by Fabregas was tipped over by Helton.
The Barclays Premier League side enjoyed a decent spell of possession, albeit without making any real impact in the final third as the first half drew to a close with Fabregas involved in some heated exchanges as the players left the field.
Arsenal started the second half well, with Rosicky looking positive down the right and having a decent shout for a penalty turned down when he seemed to be tripped.
However, disaster struck after 51 minutes following another blunder by Fabianski.
The goalkeeper contrived to pick up a back-pass from Campbell just inside the penalty area, and after realising his mistake, threw the ball to referee Martin Hansson - the Swede who missed Thierry Henry's infamous handball in France's World Cup play-off win over the Republic of Ireland.
With Campbell still complaining at the decision, Porto took the free-kick immediately - which Micael quickly tapped to Falcao for the Colombian to roll the ball into an empty net.
Wenger was left fuming on the touchline, as once again his young side were forced to drag themselves back into the game.
As the hour mark passed, Bendtner headed a free-kick over.
Fabregas removed an object which was thrown towards him when taking a corner, as the atmosphere intensified.
Uruguay defender Fucile was cautioned for a late challenge on the Arsenal skipper, which was not the first of the evening.
Arsenal settled into another decent spell but again failed to stretch the home defence.
Porto threatened on the break, but had to be content with just a one-goal lead to take back to London in a fortnight's time.