Adobe Illustrator CS6:
When I make a line with the brush tool or a squiggle, or anything, the line changes from what I drew to something ever so slightly different than what I originally had.
It also thickens and thins wherever it wants along the line, and also fills in curves with black filling.
I remember changing this and fixing it around a year ago, but now I don't remember how I did it. All I'm looking for is a nice, consistent black line. Thank you for your advice, all!