Polish state television said that Dmitri Polonski, who helped organise Russia's landgrab of the Ukrainian region, attended a conference at the Warsaw headquarters of the Organisation for Security and Co-operation in Europe on Wednesday.
The mistake was revealed when Polonski sent the media pictures of himself in the debating chamber.
The blacklist of names is in the Latin alphabet, but the Russian and Ukrainian spellings of them can sometimes be different. Polonski was travelling on his Russian passport.
Polish officials said they had not granted him a visa.
OSCE spokeswoman Lauren Baranowski confirmed that Polonski attended the debate "representing the people of Crimea".
The EU last month extended sanctions on a list of prominent Russians who are accused of playing a part in the annexation, as well as rebel leaders from eastern Ukraine.
Almost 150 people and 37 groups have had their EU assets frozen and are subject to a visa ban until March 2016.
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