This retirement misstep could cost you more than $100,000 in savings. Here's what to know.Here's what to know.

Millions of workers are making a critical mistake with their IRA rollover accounts — one that could cost them $130,000 or more in foregone investment gains, according to new Vanguard research. 

The issue stems from a quirk in the retirement system, with individual retirement accounts legally required to steer all direct contributions and most rollovers into cash, making it the "de facto default" for IRAs, Vanguard noted. That's not the case for 401(k)s, which have default options that direct workers into investments like target-date funds and the like. 

The problem, Vanguard says, is that many workers appear unaware that their IRA savings are funneled into cash, a historically poor performer when compared with equities and other investments. Keeping one's retirement assets in cash not only means that workers are losing out on the potential long-term gains, but are also eroding the value of their savings due to inflation.