Some mortgage investors got an unexpected refresher course on the risks of subprime debt when they received notice of $1 billion of previously undisclosed losses, The Wall Street Journal writes.

The unhappy surprise came with May’s monthly statements on dozens of bonds backed by 75,743 home loans made before the financial crisis to borrowers with less-than-pristine credit. Many of the losses on the $15.2 billion of loans outstanding likely weren’t reported to bondholders for a year or longer.