problem with copy / paste

i use excel 2010.
sometimes, but not always, when i do copy / paste, teh paste appears without any option, except "A".
the result is that the value is pasted but not the formula.
in the same spreadsheet, and even same cell, i can see the "behaviour" changing: sometimes "normal" copy / paste 9with all options) and sometimes only the vlue is copied.
I do not make any change intentionally.
I do not know what is causing this behaviour, or how can i rectify it.

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http://blogs.skype.com/garage/2012/0...ix_for_of.html