Hidden Load
  
Diapers. Yeah, you know what we're talkin' about if you've got kids.
In a financial sense, a load is an expense. So a "hidden load" is a charge or fee that's tucked away in the pages and pages of small-font details no one ever reads.
It comes up often when talking about investment funds, which might try to skim some additional cash from investors through buried fees. Insurance contracts are other high-profile nests of potential hidden loads.