Pasting an object with perceptual gradient applied expands it and converts into an image
- Create a rectangle and fill it with a gradient (liner/radial)
 - In the gradient panel, change the method from Classic to Perceptual
 - Copy the object
 - Paste the object Check the Appearance panel — it’d show the object is now an image, with the original perceptual fill tucked under the pixels! And if the object had a stroke, it becomes a group with a stroke in it separated.
 
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Al commented
The same problem occurs when trying to copy and paste a gradient mesh
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In my case I don’t have any external clipboard managers installed in Windows, but I do use its own Clipboard history, Win+V... disabling it does not stop the bug from happening. I tried disabling Everylang app that might steal clipboard, even though the options for this are disabled within — no luck too.
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monokano
    
 commented
    
  This issue appears to be caused by having clipboard history software installed, rather than being a bug in Illustrator.
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この問題は、Illustratorのバグではなく、クリップボード履歴ソフトウェアがインストールされていることが原因であるようです。 - 
      
A related problem:
When pasting a line with a gradient stroke it gets converted into a path
http://illustrator.uservoice.com/forums/601447/suggestions/46397521