For example, in terms of cost-bearing mistakes and their consequences, the most common include not electing available benefit coverage such as vision, dental or voluntary; choosing the wrong level of coverage; and putting too little in a flexible spending account.

As a result of paying unexpected out-of-pocket medical costs, 65 percent of workers have had to make sacrifices, including cutting back on social activities (40 percent); luxury items (34 percent); purchasing gifts (29 percent) and taking a vacation (28 percent). Others admitted to working more hours (21 percent); creating a strict household budget (21 percent) and increasing use of credit cards or line of credit (19 percent).

Seventy-four percent of workers say that when thinking about their choices for major medical insurance coverage, they only sometimes, rarely or never understand everything that is covered by their policy —while slightly more than half (59 percent) of workers who choose the same benefits year after year say they only sometimes, rarely or never have a full understanding of the changes in the policies each year.

And although most don't fully understand their health care insurance policies, workers are worried about unexpected medical expenses. In fact, 83 percent of workers say they are at least somewhat concerned about the possibility of an unexpected medical expense, with 30 percent saying they are extremely/very concerned, considering their current financial situation.

"While workers certainly need to invest more time in making better educated decisions, employers can help by understanding workers' most common mistakes, explaining their impact, and offering best-practice solutions," Tillman says.