Here is a Great Graphic of Target2 imbalances. Target2 is the real time payments settlement system in Europe. I stumbled on it looking for something else and there was no source to identify origin.
Target2 facilities cross-border movement of capital Europe. Local banks go through their central bank, while the central bank itself has an account at the ECB that is credited or debited.
The problem arises because the imbalances are getting larger and the risk of that a country leaves, or the entire union disintegrates, has risen.
If a woman in Greece takes her savings out a Greek bank and deposits them in a German bank, for example, the German bank's account at the Bundesbank is credited with the savings and the BBK account at the ECB credited. The Greek bank's account at the Greece central bank is debited and the central bank's account at the ECB is debited.
There is not limit to the imbalances. Nor is their a mechanism for final settlement. The imbalances do not seem to matter really provided the system stays together. If the system falls apart, the problems of going to be on such a magnitude that the Target2 imbalances will not be the first concern.