Manchester United, 2-0 losers to PSG, and Borussia Dortmund, hammered 3-0 by Tottenham Hotspur, have difficult tasks in the second legs of their Uefa Champions League matches this week, but they can take hope.
Even after a heavy first-leg defeat, quite a few sides have picked themselves up to progress further in the competition and take heart from some of these examples.
Four-goal deficit: In 2016/17, Barcelona became the first team to come back from four goals down to win their tie against PSG - only the fourth time it's been done in any Uefa club competition tie.
Paris's demolition of Luis Enrique's side in France was a shock; the fightback something out of this world, with Sergi Roberto striking in added time to decide the tie.
Three-goal deficits: Edin Dzzeko struggled to communicate the magnitude of Roma's achievement after his sixth-minute finish sparked an extraordinary revival of fortunes, when his side came back from 4-1 down from the first leg to take the second against Barcelona 3-0 in the 2017/18 quarterfinals.
In the 2003/04 quarterfinals, Deportivo came back from the dead, overturning a 4-1 first-leg defeat to AC Milan 4-0 in the return.
TWO-GOAL DEFICITS
Barcelona again featured in another dream comeback, this time in the 1999/2000 quarterfinals when they hammered Chelsea 5-1 (in extra time) after going 3-1 down in the first leg in London. Real Madrid 4-2 Monaco
Monaco 3-1 Real Madrid 2003/04 quarterfinals Napoli 3-1 Chelsea
Chelsea 4-1 Napoli (aet) 2011/12 round of 16 AC Milan 2-0 Barcelona
Barcelona 4-0 AC Milan 2012/13 round of 16 Olympiacos 2-0 Manchester United
Manchester United 3-0 Olympiacos 2013/14 round of 16 Paris Saint-Germain 3-1 Chelsea
Chelsea 2-0 Paris Saint-Germain 2013/14 quarterfinals Porto 3-1 Bayern Munich
Bayern Munich 6-1 Porto 2014/15 quarterfinals Wolfsburg 2-0 Real Madrid
Real Madrid 3-0 Wolfsburg..