Messing up the minus sign is a chronic issue around these parts.
In the old days, C. would say things like, "I just forgot the minus sign!" Then I would say things like, "Well, the difference between plus-70 degrees and minus-70 degrees is the difference between life and death" and we'd go from there.
The other day I came across this post to Math Forum that made me think back fondly on those days:
I have wondered if anyone knows of a notation for negative numbers
> > that would help students avoid the crippling error of thinking that
> > they are just slightly flawed positive numbers. Have you not heard
> > a student say somethings like this," the solution of 2x = -6 is
> > 3 --oh, I mean negative 3.
Any more C. seems to take minus signs more seriously. But he's still losing track of them somewhere between Point A and Point B.