Historical Linguistics and Language Families 6, Exercise 2: the Balkans

The Balkans entry lists Albanian, Romanian, southern South Slavic, Greek,
Balkan Turkish and Romani as sharing grammatical convergence, including
avoiding the infinitive and forming the future tense in similar ways. Turkish
is Turkic; the rest are Indo-European.

A shared feature is evidence of relationship only if it is inherited
through descent. Here it is shared by languages in contact, including one
from a different family, so it points to a sprachbund: features shared for
reasons other than common descent. The source warns that such features
"might give a false appearance of relatedness."

(My inference: the same reasoning is what Chapter 3 required, that
relationship needs regular correspondences in basic vocabulary, not one
shared grammatical habit.)