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Many workers who leave a job end up rolling the money from their 401(k) plan into an IRA. And while that can be a wise decision, it can backfire spectacularly if you make one crucial mistake. Nearly one-third of people — 28% — who rolled money into a Vanguard IRA in 2015 left the money sitting in cash for the next seven years, according to Vanguard research. That means those folks missed out…