Most 401(k) plans allow participants to take a loan from their account, and many workers do. An average of 13,000 401(k) participants take a loan each month for a median of about $4,600, according to an analysis of 900,000 401(k) participants by the University of Pennsylvania's Pension Research Council. About 10 percent of borrowers default on their 401(k) loans, typically due to an unanticipated job change. These loans are also subject to limits, fees and penalties. Here's what to watch out for when taking out a 401(k) loan: