Barry Eichengreen argues that:
Adopting the euro is effectively irreversible. Leaving would require lengthy preparations, which, given the anticipated devaluation would trigger the mother of all financial crises. National households and firms would shift deposits to other euro-area banks producing a system-wide bank run. Investors, trying to escape would create a bond-market crisis. Here is what the train wreck would look like.
The description is at "Eurozone breakup would trigger the mother of all financial crises."