Is mistakenly attributing something to an author that they did not say misconduct or simply bad scholarship?
Yes, intent matters, but scholars need to do due diligence in their studies. If they don't then their infractions become more serious. There are some things that scholars should know and there are some things that scholars should do. In some cases it is just sloppiness. In some cases it is intentional misconduct. 

But there is a broad area in between in which the culpability varies even if there is no evil intent in the action. 

Like anyone, a scholar needs to take care. We are trying to extend the range of what is known to be true in the world. We are also trying to give originators of ideas proper credit when they are able to advance truth. 

So, intent means a lot, but it isn't everything. And if you exercise due diligence but still err, it is just an honest mistake. Worth apologizing for, of course. 

In some fields, such as math, it is impossible for a person to know everything. That hasn't been possible for about 100 years. So, research is needed in narrow areas to make sure that you know the background of what you are doing. So, unintentional plagiarism is possible, but sloppy. 

But "sloppiness" is an infraction of its own. It isn't "pure as the driven snow". Not the worst infraction possible, but still a blot on your reputation if you do that. So "just" sloppiness might give the wrong impression.