In 1957, San Marino faced a major constitutional crisis, called the Rovereta Affair, between its two parties at the time – the Christian Democrats and the Communists. San Marino had democratically elected a Communist government in 1945, which had been re-elected since then. However, following the crushing of the Hungarian Revolution in 1956, socialist members of the Communist party split off, and all hell broke loose – with competing governments, American and Italian interventions, and the creation of armed militias on both sides. The crisis was resolved with no bloodshed, the bulk of international recognition went to the Christian Democrats’ government, and the Communist government eventually stepped down.
The Communists would come back to power for a period in the late 70s and early 80s as part of a larger governing coalition, and San Marino’s democracy hasn’t faced a constitutional crisis like Rovereta since then. However I’m sure the breakdown of functioning democracy, especially with larger powers directly interfering, still resonates in politics there today.
The above video gives an excellent detailed look at the Rovereta affair, including footage from the time.