A host of players must walk a disciplinary tightrope this week as they bid to reach the knockout stage of the competition.
However, a number of players will have to be on their best behavior in order to avoid a booking that would rule them out of the first leg of their clubs' last-16 encounters.
In Group A, Shakhtar Donetsk's Alex Teixeira and Radoslav Rakitskiy and Bayer Leverkusen's Emre Can are one caution away from a ban, with the two sides in a straight fight to join Manchester United in securing qualification for the knockout stage.
Juventus travels to Galatasaray on Tuesday needing just a point to advance from Group B, but midfielder Arturo Vidal has already picked up two yellow cards and is just one more misdemeanour shy of a suspension. The Turkish champions, meanwhile, have two players treading the disciplinary tightrope, in Albert Riera and Felipe Melo.
Three members of the Olympiakos squad, Vladimir Weiss, Kostas Manolas and Alejandro Dominguez, and the Benfica pair of Enzo Perez and Nicolas Gaitan, will also have to tread carefully to avoid triggering a ban, with both sides on seven points and battling it out for the second spot in Group C.
Bayern Munich and Manchester City have already sealed qualification from Group D, but James Milner would be well advised to keep a low profile given he's already picked up two bookings ahead of the game at the Allianz Arena.
Group E sees two teams still within a shout of joining Chelsea in the knockout stage, in Schalke and Basel, but Basel's Fabian Schar and the Schalke duo of Roman Neustadter and Marco Hoger are on the brink of a ban.
Injury means Borussia Dortmund's Sven Bender avoids the risk of picking up a third booking but star striker Robert Lewandowski could do just that as BVB travels to the Stade Velodrome for a critical match against Olympique de Marseille in Group F. Federico Fernandez of Napoli is in the same boat ahead of his side's crucial clash with Arsenal at the San Paolo.
In Group G, two Zenit St. Petersburg players, Tomas Hubocan and Igor Smolnikov, and Porto's Alex Sandro, will be suspended if booked, as will Atletico Madrid's Raul Garcia.
AC Milan hosts Ajax at San Siro on Wednesday, with a point separating the two sides in second and third place in Group H. With tensions likely to be high, Rossoneri midfielder Sulley Muntari will need to keep his competitive edge in check, as will center back Stefano Denswil for the visitors.