Defective and Mutilated Banknotes
Euro banknotes are printed on pure cotton paper which is very strong. Nevertheless, they eventually become worn and dirtied from normal use and, therefore, have to be regularly renewed by the Banco de España, which replaces them with others in good condition. In addition to this normal process of wear and tear from use, parts of banknotes or some of their security features may be lost through the action of moisture, fire, tearing, etc. All these spoilt by use banknotes should be presented at a branch of the Banco de España (or at a credit institution) for examination and possible exchange for a new one.
Articles 3 and 4 of European Central Bank Decision ECB/2003/4 
(138 KB), list the conditions and requirements for exchange of mutilated banknotes. As a general rule, a banknote will be exchanged when more than 50% of it is presented or if it can be proven that the missing part has been destroyed.
On the other hand, if, exceptionally, a banknote has a production defect, it can be exchanged under analogous conditions at any branch of the Banco de España or through a credit institution.