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Speaking of City, their build-up for the game on the training ground has, as you'd expect, been first class.

However, their manager's preparation off-the-pitch wasn't quite so hot...

Manuel Pellegrini stepped off the team bus at Manchester airport ahead of the flight to Germany to face Borussia Monchengladbach ... and realised he'd forgotten his passport.

He had to dive into a taxi and make a mad dash back to his house in Hale to fetch it, reports the Daily Mail., external

So, Chelsea ad Arsenal couldn't deliver the goods, it's now up toCityandUnitedto see what they can do.

Both need a result after defeats in their opening group games a fortnight ago.

We will be building up to tonight's games as the day goes on.

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Elsewhere in Europe, Robert Lewandowski took his recent run to 10 goals in three matches with a hat-trick as Bayern Munich beat Dinamo Zagreb 5-0 in the Champions League.

Barcelona, meanwhile, had a scare - behind at half-time to Bayer Leverkusen as they struggled without the injured Lionel Messi.

But Sergi Roberto turned in a rebound with just 10 minutes left before former Liverpool striker Suarez smashed home in the final moments. Predictable really.

We asked you for your sporting blunders and you've responded. Keep them coming to #mysportingblunder and we'll publish the best.

Chelsea face a tricky home game against Southampton on Saturday, while they come up against Dynamo Kiev in their third Group F game in the Champions League on 20 October.

Arsenal, meanwhile, face the visit of Premier League leaders Manchester United on Sunday. In the Champions League their next two matches are against Bayern Munich.

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The manager's views?

Jose Mourinho: "We watch dozens and dozens of repetitions of Porto taking corners. We were completely ready for that and in a moment when the game is under control we concede a goal that is ridiculous.

"They hit the post from a similar mistake. Apart from that, the game was balanced. Two ridiculous moments and we were punished."

Porto coach Julen Lopetegui: "The players attitude was amazing, spectacular. It was our best performance of the season. I have to congratulate my team. This three points are totally deserved." 

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So Arsenal mucked up. It was all down to Chelsea to restore pride. Oh, erm...

Jose Mourinho's men endured an equally disappointing evening in Porto, though at least the Blues already had three Champions League points in the bag after beating Maccabi Tel Aviv in their opener a couple of weeks ago.

Willian was on target, and Diego Costa hit the bar, but Maicon's flicked header proved the decisive goal.

Mourinho surprisingly left out last year's PFA player of the year Eden Hazard and on-loan striker Radamel Falcao didn't even make the bench. Problems.

Match report here.

An early view of our Champions League travails from a Sportsday reader.

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Here's a depressing fact: Prior to last night, Olympiakos had lost their previous 12 away games against English teams in European competition, scoring only three goals and conceding 37  

Arsenal boss Arsene Wenger: "It is still possible of course to qualify but it is difficult to swallow losing a game like that. We lost it on a lack of defensive concentration and bad luck. They had four shots on goal and we conceded three goals."

Asked about his decision to play Ospina ahead of Cech, the manager added: "A goalkeeper can make a mistake - that doesn't explain that we lose the game. It happened to Petr Cech already this season [against West Ham]."

And though we're not picking on David Ospina, honest, he is the inspiration for today's talking point.

We want to know this... What is the biggest blunder you've ever made on a sports field - or have seen someone else make?

Think of it as a chance to shed yourself of the weight of that dark sporting secret you've been keeping for all these years. Share it with us, we'll understand, and definitely will not take the mickey out of you. Possibly.

Tweet us now at #bbcsportsday or post a message on the BBC Sport Facebook page.

Phil McNulty
BBC Sport chief football writer

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When Arsene Wenger persuaded Chelsea to sell them goalkeeper Petr Cech, the theory behind his arrival was that the 33-year-old's experience, wisdom and acquaintance with success would make the difference in the big games.

So what sense did it make for Wenger to leave Cech on the bench and select his deputy Ospina for a game where Arsenal simply could not risk any mistakes or the concession of cheap goals?

Forget talk about any slight injury to Cech. He was fit enough to take his place on the bench so surely fit enough to start a game of this significance.

 Wenger's decision was laced with high-risk - and in the context of Arsenal's hopes of making progress in the Champions League a risk not worth taking, as well as a serious error of judgement.

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So where did it all go wrong for Arsenal? Well, it doesn't help when your keeper throws the ball in his own net, obviously (David Ospina will not be asking for a DVD of this game as a Christmas pressie).

Arsene Wenger's side knew nothing other than victory would do after defeat by Dinamo Zagreb in Croatia in their opening game - and with two meetings with group favourites Bayern Munich still to come.

But instead they endured a dreadful night and must now become the first team since Galatasaray in 2012 to lose their first two group games and still reach the knockout stage.

A tall order indeed.

Full match report here.

Which leaves things looking bleak for the Gunners in particular.

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Bit depressing this but here's Tuesday night's results. Arsenal and Chelsea fans look away now.

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