Consequences of quitting a graduate-level Research Assistant (RA) position mid-semester, with little notice?
This will likely vary greatly from one department to another. It will also depend upon your relationship with your advisor, departmental needs, etc. As @tonysdg noted in the comments, it would be a good idea to talk to your advisor about this. Some departments are understanding of these situations.

In my (geography) department, a few years ago we had a student quit his masters degree and his teaching assistantship because he was a single dad and out of money. He dreaded going to his supervisor (who was not his academic advisor) to tell him this. But it ended up being no big deal. Everyone understood, and to my knowledge there were no harsh feelings in the department toward him. Other grad students and professors covered a few classes for him through the rest of the semester, and the department was able to fund a new student the next semester.

Like I said, these things are situational, so your conversation may be different. But I included this answer as a tangible example that it is possible to leave in the middle of the semester without causing bedlam. Also, there may be strict rules against leaving in the middle of the semester, but sometimes these rules can be bent under the right circumstances. Even if you find a rule against what you're thinking about doing, I would still talk to your advisor.