It won't look pretty:
It would be a gigantic financial shockwave. Once departure by Italy were a serious prospect, there would be runs on its banks as depositors scrambled to move savings to Germany, Luxembourg or Britain, in order to avoid a forced conversion into the new weaker currency. The anticipated write-down of private and public debts, much of which is held outside Italy, would threaten bankruptcy of Europe's integrated banking system.
There would be runs on other countries that might even consider leaving. A taboo would be broken. Credit would collapse. There would be a dash for cash (those €500 euro notes would come in handy). Businesses short of it would go under. Capital controls and restrictions on travel would be needed to contain the chaos. Once the recriminations start, the survival of the European Union and its single market would be under question. It's all a frightening prospect. But that doesn't mean it won't happen.
Good thing I still have a few pesetas and escudos lying around...