A bank run can start with a rumor. Once a run has begun, it can be very hard to stop, as was described in this 1986 Los Angeles Times article.

The Department of Financial Institutions has observed that some California banks recently have been victimized by rumors. As a result, the Department issued a warning in its September Monthly Bulletin that it would take action against rumormongers. The Department noted that California Financial Code § 756 provides in relevant part...