Rule number one before playing Diego Simeone's Atletico Madrid side is not to make them angry...
But Arsenal have infuriated their Champions League opponents ahead of Tuesday night's showdown at the Emirates, who have made an official complaint to UEFA.
Atletico trained at the stadium on Monday afternoon but after working up a sweat on the turf, were unable to use their showers in the dressing room as there was no hot water, according to Marca.
It had rained during the session so Atletico opted to get on their team bus in dirty kit before travelling back to shower at the team hotel.