Jason is correct that "careless" and "sloppy" are possibilities.
"Absentminded" is another possibility.
If the person does not notice their mistake, or ignores the mistake if it is pointed out, the person is "blithe" or "oblivious". For example, one might say that someone "blithely carried on" after making a mistake. "Blithe" is a rare word; "blithely" and "oblivious" are fairly common.
"Reckless" is a legal term. Someone who is "reckless" may be held legally liable for their carelessness.
"A slip" is a technical term for a mistake where you mean to do one thing, but accidentally "slip" into a routine that results in doing something else.
The Design of Everyday Things discusses common mistakes, and how the way things are made can make it easier or harder to make a mistake.