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.
(..) to the tournament... ...need only write their name upon a piece of parchment... ...and throw it in the flame before this hour on Thursday night. Do not do so lightly. If chosen, there's no turning back. As from this moment, the Triwizard Tournament has begun. Alastor Moody. Ex-Auror... (..)2005 Harry Potter and the Goblet of Fire