There has been plenty of discussion about why Daniele de Rossi has such an unusual shirt but nobody seems to know for sure.
However the most common theory is that he'd rather wear short sleeves (to help him keep cool) but his left arm has tattoos which would displease the football authorities.
Anyway, just to prove that you weren't seeing things, here's a picture:
http://znn.india.com/...5/EURO-250612-ra2.jpg
(That photo is from the England game, so it's clear that wearing that shirt is regular practice for him).
Chris