On 29 July 2012, in the 200m freestyle qualifying heat, Nimrod Shapira Bar-On (from Israel) wore a swim cap with another person's name.
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Nimrod Shapira Bar-On came second in his qualifying heat in the 200m freestyle, Shapira Bar-On had put Jonathan Koplev‘s name on his swim cap – Koplev was supposed to participate in London-2012 Olympic but had his appendectomy burst and made him stay home (Source).
The first reason you can think about is a gesture of Nimrod Shapira Bar-On to Jonathan Koplev but surprisingly it's not the reason.
The real reason was that Nimrod didn't get a swim cap with his name and chose to wear Koplev‘s swim cap.
IOC's (Israel Olympic Association) response: "Something like this should not happen. Speedo company sent a letter with a severe reprimand demand explanations, and comprehensive investigation we will with our return to Israel. Most sincere apologies to Nimrod."
Israel Swimming Association chairman, Noam Zvi: "Speedo not made him a cap with the name. Only two days ago we realized there was a problem. We tried to organize a union cap soon, but have fallen short. "
"Speedo" response: "The company has provided sets of clothes for five swimmers that it adopts, and Shapira Bar-Or is not one of them. Speedo company does not adopt the Swimming Association and is not committed to providing swimmers equipment. Name of the swimmer Shapira Bar-Or was in the list forwarded to us from an association swimming, but since there were no dimensions, and there was no liability to pay, a cap with his name was not printed in. as a goodwill gesture Shapira Bar-Or received a late-model caps in London without a print of his name."