In Tuesday night's match with Fenerbahce, complacency could be Arsenal's enemy.
The Gunners go in to the game with a seemingly insurmountable 3-0 lead. However, it's that degree of comfort that is most dangerous to Arsenal. Arsene Wenger's side often falter when hotly favoured. Last season's shocking cup upsets against Bradford and Blackburn will not be forgotten in a hurry.
If Fenerbahce score an early goal, jitters will spread throughout the crowd and the team. Arsenal have a reputation as a team which displays occasional psychological fragility: This will be a significant test of their nerve.
Their best bet is to perform in a disciplined and professional manner, and neutralise Fenerbahce's goal threat. It's particularly important that Arsenal get to half-time with a clean sheet.
Fenerbahce's greatest threat is provided by the Senegalese striker Moussa Sow. Fenerbache will be missing Mehmet Topal, Emre Belözoglu and Joseph Yobo for the return leg at Arsenal, but Sow is eligible. The former Lille man will be tasked with firing his team back into contention for a spot in the Champions League group stage.
Despite Fenerbahce's inferiority in the first leg, Sow provided a constant threat to the Arsenal defence. His finishing was erratic but his movement was exemplary. Sow has the gift of finding space in a crowded penalty area. The ball appears to be magnetically drawn toward him, and usually he has the composure to turn those chances into goals.
Arsenal will want to field their strongest available defence to contend with Sow's off-the-ball intelligence. After his impressive performance against Fulham, Wojciech Szczesny will continue in goal.
Assuming Laurent Koscielny has recovered from the cut he suffered in the first leg, he is certain to return to play alongside stand-in captain Per Mertesacker at the heart of the back line. That combination contains positional ability, power and immaculate tackling, and will help shackle Sow.
Returning Koscielny to the centre allows Bacary Sagna to play in his preferred position at right-back. Arsenal will thus hypothetically be able to call upon their first-choice back four, although Nacho Monreal may be handed a first start of the season ahead of Kieran Gibbs. Monreal is no ordinary reserve: He is a fully fledged Spanish international, and including him will do nothing to weaken Arsenal's defensive strength.
Defensive midfield lynch-pin Mikel Arteta is still absent through injury. However, Jack Wilshere is likely to be recalled to the side to partner Aaron Ramsey, adding some steel and bite to the middle of the park. Ahead of that pair, Tomas Rosicky will hassle Fenerbahce's ball-playing centre-halves, with Theo Walcott and Santi Cazorla tasked with closing down the full-backs from out wide.
The likely inclusion of Olivier Giroud at centre-forward also helps guarantee a modicum of defensive stability. Giroud is excellent at defending set pieces, and will help keep Sow's aerial prowess under wraps.
If Arsenal can keep Sow quiet, they will almost certainly progress to the group stages. Concentration is key. With Sow around, any slip-up could prove costly.