Green’s Dictionary of Slang

1. (UK Und.) a confederate in a swindle, esp. when posing as a member of the public.

1926		Portsmouth Eve. News 23 Nov. 4/4: Mingling with the guileless folks [...] are two or three auxiliaries known as ‘ricks’ or ‘gees’. They stimulate bidding for the ‘swag’ [...] The ‘ricks’ keep a sharp eye on the bidders [at a mock-auction] to see how much they have in their note-cases.
1928		Aberdeen Jrnl 19 Dec. 8/4: ‘Ricks’ and ‘gees’ were members of the gang who mixed with the crowd, and ‘enticed the fly to walk into a web.’ To ‘sting the gee’ meant to take part in the swindle.
1936		Eve. Teleg. (Dundee) 1 Aug. 7/7: ‘Ladies and Gents I have here a case of genuine Sheffield cutlery [...] What offers?’ Here a ‘rick’ or confederate bids sixpence [...] If nobody increases the offer, another ‘rick’ calls a bob. Sooner or later a pigeon will weigh in with a bid.

2. an error, a mistake.

1956

J. Curtis Look Long Upon a Monkey 64: Done that dead to rights, just the way an N.C.O. picked up a young officer who’d went and done a rick.