The wording of the question is so ambiguous that it in fact asks 2 different questions.
has there really been only one occasion where NATO's article 5 applies?
Could mean either
- "has there been other occasions when article 5 was invoked?", or
- "has there been other occasions when the circumstances allowed for article 5 to be invoked even though the attacked country elected not to invoke it?"
The answer to the 1st question is "no, not so far" according to "Foreign Policy Research Institute."
And while there have been many times when military assistance was supplied to buttress allies' defenses, no other formal invocation has ever taken place.
The answer to the 2nd question is definitely a "Yes."
For example, the USSR shooting down of the US spy plane over Cuba could have lead to article 5 invocation because, in a private conversation, the US referred to it as "drawing the 1st blood":
Ellsberg said that Robert Kennedy (RFK) told him in 1964 that after the U-2 was shot down and the pilot killed, he (RFK) told Soviet ambassador Dobrynin, "You have drawn first blood ... . [T]he president had decided against advice ... not to respond militarily to that attack, but he [Dobrynin] should know that if another plane was shot at, ... we would take out all the SAMs and antiaircraft ... . And that would almost surely be followed by an invasion."
So it was deemed to be an act of war.
A question was raised in comments of whether Cuba would qualify under article 6.
The legal analysis in the this article states that the question of a limited area of the action of the treaty was considered.
And while the area of effect was specified by the article 6 to end at the Tropic of Cancer (a latitudinal line which passes just North of Havana), it was not meant to be a hard limit or a boundary, but rather a description of the general area where the treaty would be in effect.
the word area is to be understood to cover the general region rather than merely the North Atlantic Ocean in a narrow sense. This is also confirmed in the 1949 Executive Report on the North Atlantic Treaty produced by the U.S. Senate Committee on Foreign Relations. In it, the U.S. Senate stressed that “[i]n view of the purpose of the treaty to deter armed attack, the area covered by the treaty was deliberately described in general terms rather than defined by the lines of a map.
Lastly, since the retaliation described by RFK would have clearly been a massive military action, it would have been to the benefit of the European NATO allies to be prepared for a European escalation by the USSR. So invocation of the article 5 would have been in line with the gravity of what would be about to happen.