http://en.wikipedia.org/wiki/Distress_signal
Distress signal
For hundreds of years inverted national flags (were) are commonly used as distress signals. It is difficult or impossible to determine when many countries (such as those of Argentina or Italy) are inverted, or the viewer may not be familiar with the flag and may not realise it is not upright. If a national flag is available, distress may be indicated by flying it upside down
