This comment on a site offering tips to writers brought me to a stop:
Does your writing suffer from waaaay too many em-dashes? Incareful authors often use them in place of a comma…
The standard negative form of careful is careless: “full of care,” and “without care.”
If a writer had a reason to form the negative with a prefix, the one to use with careful would be –un, not –in. With a few exceptions, the negative prefix -in is used with words of Latin origin. The word care is from Old Engish carian, cearian “be anxious, grieve; to feel concern or interest.”
But, there’s no need to bother with “uncareful” because we already have the word careless.
Here are some examples of the form “incareful” infesting the web:
But often enough, incareful work causes hazards.
The configuration scheme is infinitely flexible, and apparently has enough sharp edges for the incareful geek to hurt himself upon.
However, incareful reading can lead the impression that there are inconsistencies,
There is little or no opportunity to save what might be a good and even ground-breaking proposal sunk by the inexperience or incareful reviewing of one person.
…people who were incareful enough to get caught doing what a lot of men do every day”.
Most of the time a man will feel incareful if he is being unwanted in any way. (This writer may have been reaching for “uncared for.”)
I suspect that the popularity of the nonword “incareful” may have something to do with an unconscious feeling that it sounds less judgmental than careless.
Some synonyms for careless: