This can occur when a company decides to suspend dividend payments during tough financial times, as we saw with several companies during the 2008 financial crisis.
As a current example, Goodrich Petroleum Corporation suspended preferred dividends on three different series of its preferred stock in August 2015 and made it clear in a press release that the unpaid dividends will accumulate. If a similar situation occurs with any preferred stocks you own, here's how to calculate the cumulative dividends owed to you.