Probably the most notable incident of this type was a DL 777 flight out of JNB which put down at ASI in the South Atlantic with an engine out a few years back. Technically ASI is British territory but with a small joint US / UK base. IIRC civilians are not usually permitted on the island without special permission / visa, there is no indigenous population. Must have caused chaos when best part of 300 pax & crew turned up with little warning.
Read on another thread that ASI is now closed to heavies due to 'runway problems' so wonder what that does to flight planning for such ETOPS operations