Actually, the example is wrong.
Sauerstoffflasche is a composite word, `Sauerstoff' meaning oxygen and `Flasche' bottle.
There is a rule that ligatures can't connect parts of composite words, so the correct orthography is: Sauerstoffflasche, ff and fl being separate ligatures.
There is another crazy German example for the same problem: the meanings of the words `Wachstube' and `Wachstube' are different! (st == an st ligature.) The first one, Wachs-Tube, means waxtube, while Wach-Stube is guardroom.