This is a problem that should not be 'taken lightly'. [=this is a problem that should be treated seriously]
Considering the harm he did, he's 'gotten off lightly' with only a warning.
Our troops were armed too 'lightly' to take on the heavily armed invaders.
(..) Just stop. Stop. Okay, they're stopped. I need you to walk back exactly the way we came from. Very, very calmly, okay? Tread lightly. Heel, toe. Doing good. Doing great. Very slowly. Good. Come on. That's it. That's great. (..)2008 Journey to the Center of the Earth