Acting without thinking things through is carelessness. Don't try to text a friend while you're riding your bike, or your carelessness may send you crashing into a mailbox.
You can think of carelessness as negligence, a failure to pay close attention or carefully consider the possible ramifications of your actions. Driver carelessness causes many traffic accidents, and a writer's carelessness can result in grammatical errors and misspellings. The Old English version of this word, carleasnes, meant "freedom from anxiety," something closer to nonchalance than today's carelessness.