combine straight border with the last border having a mitered corner?
 

I have a quilt that I would like to do the last border and miter it because it has stripes. Is this one of the cardinal rules not to do? The other two borders were done with straight borders.

No reason you can't miter this last one separately from the already completed borders. I agree, a stripe fabric looks best done with a miter. The only cardinal rule I know about mitering borders is that if you are going to do ALL of them that way they should be sewn together first then mitered as one unit.

Thank you for your response and yes I do know that I sew the borders together before mitering them to the quilt.

I think it would look fine.

If I was mitering a stripped fabric I would have to keep lines and colours matching. Just my OCD thing. I would do it slowly. And carefully .

It is your quilt, your time and money spent. So in my book, you can do as you please. I think a mitered corner would look great.

I do both straight and mired borders. However the flat or straight are solid fabrics or fabrics that appear solid. I believe two patterned fabric would look disjointed