Amanda Bynes -- Accused of Shoplifting in Fancy NYC Store

Amanda Bynes was accused of going full Winona Ryder -- she was caught by security at a trendy New York City store allegedly trying to steal merchandise. NYC cops were called ... but we've learned they just let her go.

According to our sources, Bynes was nabbed by security guards at Barneys on Madison Ave. after she tried to leave without paying. Law enforcement sources tell us she put on a $200 hat and walked out of the store ... security nabbed her outside.

We're told they brought Bynes back in, where witnesses say she was "acting crazy." 4 NYPD officers arrived on scene to question her, but security had already let her go.

Our sources say ... Bynes told security guards she was being chased by fans inside Barneys ... so she grabbed the cap to cover her face. She said she had no intention of stealing anything ... so they let her go. Barneys made her sign a no-trespass agreement ... banning her from re-entering the store.