[loeg]

Pretty much any year.

http://awealthofcommonsense.com/2016/05/deconstructing-30-ye...

> The worst 30 year return — using rolling monthly performance — occurred at the height of the market just before the Great Depression and stocks still returned almost 8% per year over the ensuing three decades.

(This number is not adjusted for inflation, but still.)

https://www.thebalance.com/rolling-index-returns-1973-mid-20...

> The S&P 500 Index, shown in bright red, delivered its worst twenty-year return of 6.4% a year over the twenty years ending in May 1979.