One swallow doesn't make a summer
A single instance of something is just that; it doesn't indicate a trend.
This old proverb is listed in several early glossaries, notably Richard Taverner's transcription of the [Latin] proverbs of Erasmus - Prouerbes or adagies with newe addicions, gathered out of the Chiliades of Erasmus, 1539:
It is not one swalowe that bryngeth in somer. It is not one good qualitie that maketh a man good.
It is interesting that, even in the 16th century, the figurative meaning of the phrase was made explicit.
See also: the List of Proverbs.
Other 'One' phrases: