I'll note here that the Morales' plane story is much more complicated that it's made to be in the other answer. It was apparently a false flag operation to a good extent, misleading the Westerners as to whom was on-board; or at least Assange claimed he did that. Anyway, the Morales plane landed in Austria because its crew claimed it could not read its fuel gauges:
Control tower: Do you need any assistance?
Pilot: Not at this moment. We need to land because we cannot get a correct indication of the fuel indication so as a precaution we need to land.
Why the plane landed in Austria isn't clear, but it doesn't seem to have been at the insistence of the Austrians.
Then Morales declared outrage for being "forced" to land.
The issue whether the plane was actually denied airspace entry (and by whom) is a bit more murky. At least France clearly apologized, but Spain the plane was actually due to land for refueling (Canary Islands, actually) never made a clear statement. It is known that Portugal granted overflight some days before, but refused the request to refuel on their territory. (I'll spare you the quotes on that.) So it's not really a clear-cut situation like a jet fighter showing up and requesting the plane land.
Also the reports whether the Morales' plane was searched are fairly contradictory. An Austrian minister initially said yes, but after Bolivians denied that took place, the details came out as:
"Someone from the airport staff sought out the aircraft or the pilot after landing to inquire about the nature of the technical problem," [Austrian President Heinz] Fischer was quoted as saying.
"The Austrian official was advised that the defect was already fixed, and saw on this occasion that the plane was empty... He did not look under the seats. There was no formal inspection, but no other people were found on board," Fischer added.
Pressed on whether that meant Austrians had not searched the plane, he said, "There was no search in the forensic sense. There was also no reason to under international law. The plane of a president belongs to 'his territory' and cannot be searched readily."
N.B. Assange was aware of this too, as in the interview (first link) he said:
from a legal perspective, they are flying embassies.
On the other hand, that does not mean countries have to allow them:
Eurocontrol - which co-ordinates Europe's airspace and traffic control - said it was "a national decision whether or not to accept a state flight" under the terms of the Chicago Convention on International Civil Aviation.
But there's a [legal] difference between not allowing a flight to enter and forcing it to land.