Billions may have been lost by investors that bought a commodity index basket as a inflation hedge or a form of asset class investment. This was based on a correlation with inflation that was not very strong and on the believe that the roll yield from backwardation would always exist.
There was the turn in the commodity super-cycle that worked against investors but the real investment widow-maker in commodities was the failure to properly account for the significant shift to contango from backwardation. If demand declines and inventories are high, the contango will be a significant drag on returns.
The commodity income was a key driver for returns over the long-run. Because investment in commodities is through futures, this carry component is critical to successful investing.