In 1992 Tatarstan held a referendum on independence from Russia, and 62 percent of the those who took part voted in favor of independence. On February 15, 1994 the Treaty On Delimitation of Jurisdictional Subjects and Mutual Delegation of Authority between the State Bodies of the Russian Federation and the State Bodies of the Republic of Tatarstan and Agreement between the Government of the Russian Federation and the Government of the Republic of Tatarstan (On Delimitation of Authority in the Sphere of Foreign Economic Relations) were signed. These agreements may be considered as temporary recognition of Tatarstan's independence by the Russian Federation, because it mentions the Declaration on State Sovereignty of the Republic of Tatarstan.
-From Wiki.
What if Tatarstan refused to sign the aforementioned document with an extremely long name and became independent? Russia wasn't very powerful at this time, right after the fall of the Soviet Union, so it couldn't have done a ton to stop this. This might have also spurred all the other Federal Subjects of Russia, such as Chechnya, North Ossetia-Alania, Ingushetia, Sakha, Altai, Dagestan, Karelia, and the Jewish Autonomous Oblast, to become independent, or less cooperative with Russia. And with Sakha, which even today does not cooperate well with Russia's federal government, if it left Russia, Russia would have lost one heck of a lot of land.