Careless, heedless, thoughtless, inadvertent mean showing lack of concern or attention.
Careless often implies the absence of such cares as responsibilities or worries; it then usually connotes casualness, spontaneity, and lightheartedness and carries little or no suggestion of culpability.
Often, however, the term implies a more or less culpable indifference which at its best is the product of independence or of concentration on other and more important things and at its worst is the result of laziness or negligence and manifests itself in blameworthy lack of pains or thought.
Heedless also implies indifference, but it stresses inattentiveness or a failure to see, observe, take note of, or remark rather than laziness or negligence; the term often also connotes lightmindedness, frivolousness, or flightiness.
Thoughtless may emphasize lack of reflection or of forethought. More frequently it suggests lack of thoughtfulness or consideration for others.
Inadvertent usually implies heedlessness; the term is rarely applied to persons or their minds but is used in qualifying their acts and especially such of their mistakes, errors, or blunders as ensue from heedlessness or inattention resulting from concentration on other things rather than from ignorance or intention.