Simple:
* He was careless with his belongings and lost his keys.
* The careless driver caused a minor accident.
* Her careless words hurt his feelings.
More Complex:
* The company's careless handling of sensitive data led to a major security breach.
* Despite her best intentions, she made a careless mistake that cost the team the game.
* The careless use of firecrackers ignited a brush fire.
Figurative:
* He was a careless artist, allowing his emotions to guide his brushstrokes.
* The careless whispers of the wind carried secrets through the trees.
Remember that "careless" can be a negative adjective, implying negligence or lack of attention.