Same problem here with WP 3.5.1:
"...wp-includes/SimplePie/Item.php on line 1490"
Some days ago I deactivted and activated all plugins and the error message was gone but now it is back :-(
It happens only to one of my four similar built web pages, this is strange. I have to check if there is a plugin which causes it and the plugin is not used in the other pages!?
EDIT after only 5 minutes only (that was fast):
I deavtivated NextGen Gallery 1.9.12 and the error message was gone.
Now I have just to change my links to my pics.
Anyway I use Picasaview now for the big gallery.....