The Spanish midfielder, on loan from Espanyol, celebrated his 24th birthday on April 15 and had a friend coming over to visit from Spain.
Riera drove down to London to meet his pal and spotted him coming through arrivals. However, Riera's mate was stopped by customs staff because he had carried a small dog in in his hand luggage.
Riera spotted the commotion and got into an argument with security staff which resulted in him being arrested and fined almost £100 for a public order offence.
It's not known what happened to the dog...
Source: www.uefa.com