The warning relates to a chemical reaction if the handgrip changes colour which could cause an allergic reaction
http://www.dpreview.com/news/2012/07/10/canon-warns-of-allergen-in-650d-T4i-grip
http://www.dpreview.com/news/2012/07/10/canon-warns-of-allergen-in-650d-T4i-grip